* ext/fiddle/win32/libffi.mk.tmpl: assemble without directory prefix.
workaround of a bug of VC12 ml, by unak at [ruby-core:67792].
[ruby-core:67789] [Bug #10780]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49397 b2dd03c8-39d4-4d8f-98ff-823fe69b080e