symbol.c: move macros

* symbol.c (SYMBOL_PINNED, SYMBOL_PINNED_P, ID_DYNAMIC_SYM_P),
  (STATIC_SYM2ID, STATIC_ID2SYM): move from symbol.h as these
  macros are used only in symbol.c.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47058 b2dd03c8-39d4-4d8f-98ff-823fe69b080e