TARGET_MCU_NRF51822 - Replace definition of SVCALL with the one used by SDK V11.

This change avoid warnings about unused functions.

Author: pan-

features/FEATURE_BLE/targets/TARGET_NORDIC/TARGET_MCU_NRF51822/sdk/source/softdevice/s130/headers/nrf_svc.h

@@ -42,11 +42,11 @@
 #define SVCALL(number, return_type, signature) return_type __svc(number) signature
 #elif defined (__GNUC__)
 #define SVCALL(number, return_type, signature) \
-  _Pragma("GCC diagnostic ignored \"-Wunused-function\"") \
-  _Pragma("GCC diagnostic ignored \"-Wunused-parameter\"") \
   _Pragma("GCC diagnostic push") \
   _Pragma("GCC diagnostic ignored \"-Wreturn-type\"") \
-  __attribute__((naked)) static return_type signature \
+  __attribute__((naked))                                \
+  __attribute__((unused))                               \
+  static return_type signature                          \
   { \
     __asm( \
         "svc %0\n" \