Add third_party/libmtp/ to .gitignore.

This was added to DEPS by r149485.

BUG=none
TEST=none
TBR=thestig@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10825144

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@149526 0039d316-1c4b-4281-b951-d872f2087c98

.gitignore

@@ -164,6 +164,7 @@ v8.log
 /third_party/libexif/sources
 /third_party/libjingle/source
 /third_party/libjpeg_turbo
+/third_party/libmtp/
 /third_party/libphonenumber/libphonenumber.xml
 /third_party/libphonenumber/libphonenumber_without_metadata.xml
 /third_party/libphonenumber/src