Fix travis macos+scons build.

Steps:
1) added `brew install libffi`, was told it was already installed and to try reinstalling.
2) changed to `brew reinstall libffi`, was told it was "keg only" and was provided the PKG_CONFIG export statement.
3) added export statement, seems to work.

utils/travis/steps/install.sh

@@ -24,6 +24,7 @@ if [ "$TRAVIS_OS_NAME" = "osx" ]; then
         travis_wait ./projectfiles/Xcode/Fix_Xcode_Dependencies
     else
         brew install scons cairo pango moreutils sdl2_image sdl2_ttf sdl2_mixer glew ccache
+        brew reinstall libffi
     fi
 else
     if [ "$NLS" != "true" ]; then

utils/travis/steps/script.sh

@@ -26,6 +26,7 @@ if [ "$TRAVIS_OS_NAME" = "osx" ]; then
         "$CXX" --version
         if [ "$TOOL" = "scons" ]; then
             ln -s $HOME/build-cache/ build
+            export PKG_CONFIG_PATH="/usr/local/opt/libffi/lib/pkgconfig"
 
             scons wesnoth wesnothd campaignd boost_unit_tests build=release \
                   ctool="$CC" cxxtool="$CXX" cxx_std="$CXXSTD" \