mkmf.rb: -I flags to $INCFLAGS

* lib/mkmf.rb (MakeMakefile#pkg_config): separate -I flags to
  $INCFLAGS, which is used by CPP.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47458 b2dd03c8-39d4-4d8f-98ff-823fe69b080e