BMX280 defines must be guarded

[0x3333]

Some BMX280 sensor defines in sensors.h aren't guarded by #ifdef, when defining them in custom.h for example generates a warning.

Defines: BMX280_MODE, BMX280_STANDBY, BMX280_FILTER, BMX280_TEMPERATURE, BMX280_PRESSURE.
...

#ifndef BMX280_NUMBER
#define BMX280_NUMBER                   1       // Number of sensors present. Either 1 or 2 allowed
#endif
#ifndef BMX280_ADDRESS
#define BMX280_ADDRESS                  0x00    // 0x00 means auto (0x76 or 0x77 allowed) for sensor #0
#endif                                          // If (BMX280_NUMBER == 2) and
                                                //   (BMX280_ADDRESS == 0x00) then sensor #1 is auto-discovered
                                                //   (BMX280_ADDRESS != 0x00) then sensor #1 is the unnamed address

#define BMX280_MODE                     1       // 0 for sleep mode, 1 or 2 for forced mode, 3 for normal mode
#define BMX280_STANDBY                  0       // 0 for 0.5ms, 1 for 62.5ms, 2 for 125ms
                                                // 3 for 250ms, 4 for 500ms, 5 for 1000ms
                                                // 6 for 10ms, 7 for 20ms
#define BMX280_FILTER                   0       // 0 for OFF, 1 for 2 values, 2 for 4 values, 3 for 8 values and 4 for 16 values
#define BMX280_TEMPERATURE              1       // Oversampling for temperature (set to 0 to disable magnitude)
                                                // 0b000 = 0 = Skip measurement
                                                // 0b001 = 1 = 1x 16bit/0.0050C resolution
                                                // 0b010 = 2 = 2x 17bit/0.0025C
                                                // 0b011 = 3 = 4x 18bit/0.0012C
                                                // 0b100 = 4 = 8x 19bit/0.0006C
                                                // 0b101 = 5 = 16x 20bit/0.0003C
#define BMX280_HUMIDITY                 1       // Oversampling for humidity (set to 0 to disable magnitude, only for BME280)
                                                // 0b000 = 0 = Skip measurement
                                                // 0b001 = 1 = 1x 0.07% resolution
                                                // 0b010 = 2 = 2x 0.05%
                                                // 0b011 = 3 = 4x 0.04%
                                                // 0b100 = 4 = 8x 0.03%
                                                // 0b101 = 5 = 16x 0.02%
#define BMX280_PRESSURE                 1       // Oversampling for pressure (set to 0 to disable magnitude)
                                                // 0b000 = 0 = Skipped
                                                // 0b001 = 1 = 1x 16bit/2.62 Pa resolution
                                                // 0b010 = 2 = 2x 17bit/1.31 Pa
                                                // 0b011 = 3 = 4x 18bit/0.66 Pa
                                                // 0b100 = 4 = 8x 19bit/0.33 Pa
                                                // 0b101 = 5 = 16x 20bit/0.16 Pa
 

...

I'll create a PR to fix this.

[0x3333]

Added PR #1867 with fix.