Fix -Wold-style-definition

http://ci.rvm.jp/results/trunk-yjit@ruby-sp2-docker/4612329

Also, I changed the position of `{` to be consistent with the rest of
the codebase.

yarp/yarp_init.c

@@ -3,6 +3,7 @@
 void ruby_init_ext(const char *name, void (*init)(void));
 
 void
-Init_YARP() {
+Init_YARP(void)
+{
     ruby_init_ext("yarp.so", Init_yarp);
 }