Nordic: Enabled the invocation of twi_master.h functions from a cpp context

In a previous commit, the content of twi_master.h did not allow the invocation
of the c functions specified by the file from a cpp context. This commit wraps
the contents of the header file with the appropriate ifdefs.

libraries/mbed/targets/hal/TARGET_NORDIC/TARGET_MCU_NRF51822/twi_master.h

@@ -13,6 +13,11 @@
 #ifndef TWI_MASTER_H
 #define TWI_MASTER_H
 
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /*lint ++flb "Enter library region" */
 
 #include <stdbool.h>
@@ -95,5 +100,9 @@ bool twi_master_transfer(uint8_t address, uint8_t *data, uint8_t data_length, bo
  *@}
  **/
 
+#ifdef __cplusplus
+}
+#endif
+
 /*lint --flb "Leave library region" */
 #endif //TWI_MASTER_H