Add main.c and update CI

.gitea/workflows/ci.yml

@@ -5,5 +5,6 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - name: Printing hello
-        run: echo "Hello, 🌍"
+      - uses: actions/checkout@v4
+      - name: Build using gcc
+        run: gcc -Wall -Werror main.c

main.c

@@ -0,0 +1,5 @@
+int main()
+{
+    printf("Halo, Welt!");
+    return 0;
+}