Allow certain modules APIs only defining REDISMODULE_EXPERIMENTAL_API.

Those calls may be subject to changes in the future, so the user should
acknowledge it is using non stable API.

src/modules/helloblock.c

@@ -31,6 +31,7 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
+#define REDISMODULE_EXPERIMENTAL_API
 #include "../redismodule.h"
 #include <stdio.h>
 #include <stdlib.h>