mbed_fault_handler: fix build warning

Fixes warning

BUILD\NRF52840_DK\GCC_ARM\mbed-os\cmsis\TARGET_CORTEX_M\mbed_fault_handler.o
.\mbed-os\cmsis\TARGET_CORTEX_M\mbed_fault_handler.c
[Warning] mbed_fault_handler.c@149,5: implicit declaration
of function 'memcpy' [-Wimplicit-function-declaration]

Author: Naveen Kaje

cmsis/TARGET_CORTEX_M/mbed_fault_handler.c

@@ -18,6 +18,7 @@
 #define __STDC_FORMAT_MACROS
 #endif
 #include <inttypes.h>
+#include <string.h>
 
 #include "device.h"
 #include "mbed_error.h"