on most of platforms except mswin.

Changing it to static was needed for mswin because it doesn't use
transform_mjit_header, but for platforms that use it, it causes link
error like:

/usr/bin/ld: /tmp/_ruby_mjit_p21652u0.o: relocation R_X86_64_PC32 against undefined symbol `rb_vm_search_method_slowpath' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: Bad value
collect2: error: ld returned 1 exit status

include/ruby/defines.h: MJIT_FUNC_EXPORTED moved to intern.h

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64942 b2dd03c8-39d4-4d8f-98ff-823fe69b080e