sdl12-compat: CI: Need GLU headers for testgl.c, too.

SDL Commits
1 
From dae50d7df330725a34f4cbd5da58e6537f0d5425 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Tue, 1 Jun 2021 20:44:07 -0400
Subject: [PATCH] CI: Need GLU headers for testgl.c, too.

---
 .github/workflows/main.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 24c63bf..f46387b 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -17,7 +17,7 @@ jobs:
       if: runner.os == 'Linux'
       run: |
         sudo apt-get update
-        sudo apt-get install cmake ninja-build libgl1-mesa-dev
+        sudo apt-get install cmake ninja-build libgl1-mesa-dev libglu1-mesa-dev
     - name: Get sdl12-compat sources
       uses: actions/checkout@v2
     - name: Get SDL2 headers