cast void data pointer to const char * to fix gcc

Former-commit-id: a78169e9e043aea83c3f8b8fbb14e605575f8575

src/modules/hellotimer.c

@@ -40,7 +40,7 @@
 /* Timer callback. */
 void timerHandler(RedisModuleCtx *ctx, void *data) {
     REDISMODULE_NOT_USED(ctx);
-    printf("Fired %s!\n", data);
+    printf("Fired %s!\n", (const char *) data);
     RedisModule_Free(data);
 }