travis: fix osx make build

This variable was removed in commit 087af992a276e7478c9c "travis: remove
unused linux code path" because it looked like it was only used by the
Linux build. Turns out I was wrong, so let's restore it.

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@chromium.org>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
diff --git a/.travis.yml b/.travis.yml
index f061ad5..96235a7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,6 +5,10 @@
 cache:
   ccache: true
 
+env:
+  global:
+    - PKG_CONFIG_PATH="$PKG_CONFIG_PATH"
+
 matrix:
   include:
     - env: