gc.c: fix build for testing w/o RGenGC

* gc.c (ruby_gc_set_params): fix building without RGenGC

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45224 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* ext/readline/readline.c (Init_readline): Use rl_hook_func_t instead
  of Function to support readline-6.3.  (rl_hook_func_t is available
  since readline-4.2.)
  Reported by Dmitry Medvinsky.  [ruby-core:61141] [Bug #9578]



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45225 b2dd03c8-39d4-4d8f-98ff-823fe69b080e