Fix return type of callback function to rb_thread_call_with_gvl().

The return type is not used but caused a compiler warning:
  Function.c: In function 'callback_invoke':
  Function.c:479:9: warning: passing argument 1 of 'rb_thread_call_with_gvl' from incompatible pointer type [enabled by default]
  Function.c:102:14: note: expected 'void * (*)(void *)' but argument is of type 'VALUE (*)(void *)'

This addresses issue #297.
Merge pull request #309 from larskanis/fix-return-of-callback_with_gvl

Fix return type of callback function to rb_thread_call_with_gvl().