target: Include missing `cmsis_nvic.h`

The header `cmsis_nvic.h` defines vector start address in RAM
`NVIC_RAM_VECTOR_ADDRESS` which is used in
`mbed_boot.c:mbed_cpy_nvic()`. But `mbed_boot.c` only includes
`cmsis.h`. Due to this `mbed_cpy_nvic` becomes an empty function and the
vectors don't get relocated to RAM. This causes BusFault error when Mbed
OS tries to update any of the IRQ handlers.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>

targets/TARGET_ARM_SSG/TARGET_MUSCA_B1/device/cmsis.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017-2019 Arm Limited
+ * Copyright (c) 2017-2020 Arm Limited
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -81,4 +81,8 @@
   #warning Not supported compiler type
 #endif
 
+#if defined(TARGET_MUSCA_B1)
+#include "cmsis_nvic.h"
+#endif
+
 #endif /*__MUSCA_B1_CMSIS_H__ */