macosx auto-build: install pkg-config before attempting to build

.github/workflows/build_macosx.yml

@@ -15,7 +15,7 @@ jobs:
 
     - name: install dependencies
       run: |
-        brew install qt@5
+        brew install qt@5 pkg-config
         brew link qt@5 --force
         git clone https://github.com/FreeSpacenav/libspnav
         cd libspnav