Added GitHub actions OS X build

.github/workflows/osx.yml

@@ -0,0 +1,22 @@
+name: OS X
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  build:
+    name: QtPBFImagePlugin
+    runs-on: macos-10.15
+    steps:
+      - name: Set environment variables
+        run: echo "PATH=/usr/local/opt/qt@5/bin:$PATH" >> $GITHUB_ENV
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Install dependencies
+        run: brew install qt5 protobuf
+      - name: Configure build
+        run: qmake PROTOBUF=/usr/local pbfplugin.pro
+      - name: Build project
+        run: make -j3