Add /* fallthrough */ to fix -Wimplicit-fallthrough warning

rts/sm/Scav.c

@@ -1722,9 +1722,11 @@ scavenge_static(void)
 
     case FUN_STATIC:
       scavenge_fun_srt(info);
-      // fallthrough: a FUN_STATIC can also be an SRT, so it may have pointer
-      // fields.  See Note [SRTs] in CmmBuildInfoTables, specifically the [FUN]
-      // optimisation.
+      /* fallthrough */
+
+      // a FUN_STATIC can also be an SRT, so it may have pointer
+      // fields.  See Note [SRTs] in CmmBuildInfoTables, specifically
+      // the [FUN] optimisation.
 
     case CONSTR:
     case CONSTR_NOCAF: