libgtk3: ensure host version of pkg-config is used for native compilation

libgtk3: ensure host version of pkg-config is used for native compilation

This already was in Eric's submission, but I removed it when committing
while cleaning up.  The package infrastructure already passes the correct
pkg-config to use for cross compilation, but NOT for building native tools,
so we ended up using the target variant and got wrong libraries/headers.

Fixes:

http://autobuild.buildroot.net/results/e0c/e0c2623e2d400ce66963eae23f136c0158556b36/
http://autobuild.buildroot.net/results/5bf/5bf66b8d16a3ddded24827bf278ad24b55ded6e7/
http://autobuild.buildroot.net/results/75f/75f93cea8ab59f052ddefa51c0bfc14c3df47a60/

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>