Revert "* internal.h: prototype declarations of gc.c collect up."

These declarations should be exported.

This reverts commit 3c9f88c2ce138f2714c36cd25dc9b50ec93861e7.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43611 b2dd03c8-39d4-4d8f-98ff-823fe69b080e