[GTK] Webkit fails to build webkit2gtk-tests-resources.gresource.
https://bugs.webkit.org/show_bug.cgi?id=114485

Patch by Hanyee Kim <choco@company100.net> on 2013-04-12
Reviewed by Martin Robinson.

Generating webkit2gtk-tests-resources.gresource can be failed due to
the non-existing target directory.
We need to make target directory before generating it.

* UIProcess/API/gtk/tests/GNUmakefile.am:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148279 268f45cc-cd09-0410-ab3c-d52691b4dbfc