Make gitignore for *.mk more specific.
Instead of ignoring *.mk, only ignore *.host.mk and *.target.mk, which
is what the gyp-generated makefiles are actually called. The android
port has a number of files called things like Android.mk which are
hand-written, not autogenerated, and ignoring them causes them to be
left out of commits by mistake sometimes.
BUG=
Review URL: https://chromiumcodereview.appspot.com/
10882030
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@153897 0039d316-1c4b-4281-b951-d872f2087c98