cm3/sync.h: Add DECLS to allow use with c++

Reported by vvirag on IRC. Diagnosed and fixed by zyp on IRC

include/libopencm3/cm3/sync.h

@@ -22,6 +22,8 @@
 
 #include "common.h"
 
+BEGIN_DECLS
+
 void __dmb(void);
 
 /* Implements synchronisation primitives as discussed in the ARM document
@@ -52,4 +54,6 @@ void mutex_unlock(mutex_t *m);
 
 #endif
 
+END_DECLS
+
 #endif