Fix compiler warning: "UNUSED" redefined

Fix #44
cores/arduino/syscalls_stm32.c:45:0: warning: "UNUSED" redefined
 #define UNUSED(x) x ## _UNUSED __attribute__((__unused__))

Signed-off-by: Frederic.Pillon <[email protected]>

Author: fpistm

cores/arduino/syscalls_stm32.c

@@ -41,6 +41,9 @@
 
 // Helper macro to mark unused parameters and prevent compiler warnings.
 // Appends _UNUSED to the variable name to prevent accidentally using them.
+#ifdef UNUSED
+#undef UNUSED
+#endif
 #ifdef __GNUC__
 #define UNUSED(x) x ## _UNUSED __attribute__((__unused__))
 #else