rts: Work around missing prototypes errors

Darwin's toolchain inexpliciably claims that `write_barrier` and friends
have declarations without prototypes, despite the fact that (a) they are
definitions, and (b) the prototypes appear only a few lines above. Work
around this by making the definitions proper prototypes.

(cherry picked from commit 5b6612bc)

rts/include/stg/SMP.h

@@ -589,9 +589,9 @@ load_load_barrier(void) {
 EXTERN_INLINE void write_barrier(void);
 EXTERN_INLINE void store_load_barrier(void);
 EXTERN_INLINE void load_load_barrier(void);
-EXTERN_INLINE void write_barrier     () {} /* nothing */
-EXTERN_INLINE void store_load_barrier() {} /* nothing */
-EXTERN_INLINE void load_load_barrier () {} /* nothing */
+EXTERN_INLINE void write_barrier     (void) {} /* nothing */
+EXTERN_INLINE void store_load_barrier(void) {} /* nothing */
+EXTERN_INLINE void load_load_barrier (void) {} /* nothing */
 
 // Relaxed atomic operations
 #define RELAXED_LOAD(ptr) *ptr