Install libgl-dev on Ubuntu instances of CI

Author: rexim

.github/workflows/ci.yml

@@ -6,6 +6,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v1
+      - name: install dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -qq libgl-dev
       - name: build sowon
         run: |
           make
@@ -15,6 +19,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v1
+      - name: install dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -qq libgl-dev
       - name: build sowon
         run: |
           make