#5 only set I2C_BUFFER_LENGTH if not already defined

[trycoon]

only set I2C_BUFFER_LENGTH if not already defined, this fixes compile warnings on ESP32:

Compiling .pioenvs/nodemcuv2/lib3cb/SparkFun Ublox Arduino Library_ID5746/SparkFun_Ublox_Arduino_Library.cpp.o
In file included from .piolibdeps/SparkFun Ublox Arduino Library_ID5746/src/SparkFun_Ublox_Arduino_Library.cpp:29:0:
.piolibdeps/SparkFun Ublox Arduino Library_ID5746/src/SparkFun_Ublox_Arduino_Library.h:71:0: warning: "I2C_BUFFER_LENGTH" redefined
#define I2C_BUFFER_LENGTH 32
^
In file included from .piolibdeps/SparkFun Ublox Arduino Library_ID5746/src/SparkFun_Ublox_Arduino_Library.h:38:0,
from .piolibdeps/SparkFun Ublox Arduino Library_ID5746/src/SparkFun_Ublox_Arduino_Library.cpp:29:
/home/trycoon/.platformio/packages/framework-arduinoespressif32/libraries/Wire/src/Wire.h:34:0: note: this is the location of the previous definition
#define I2C_BUFFER_LENGTH 128

[nseidle]

Excellent. Thank you!