Fix compilation of smp.h with C++ and disabled SMP

Signed-off-by: Paul Guyot <pguyot@kallisys.net>

Author: pguyot

src/libAtomVM/smp.h

@@ -273,6 +273,10 @@ bool smp_is_main_thread(GlobalContext *glb);
 #define SMP_RWLOCK_WRLOCK(lock) smp_rwlock_wrlock(lock)
 #define SMP_RWLOCK_UNLOCK(lock) smp_rwlock_unlock(lock)
 
+#ifdef __cplusplus
+}
+#endif
+
 #else
 
 #define SMP_SPINLOCK_LOCK(spinlock)
@@ -287,8 +291,4 @@ bool smp_is_main_thread(GlobalContext *glb);
 #define SMP_RWLOCK_UNLOCK(lock)
 #endif
 
-#ifdef __cplusplus
-}
-#endif
-
 #endif