Added new program, updated workflow

.gitea/workflows/ci.yaml

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

main.c

@@ -0,0 +1,4 @@
+int main() {
+  printf("Halo, welt!");
+  return 0;
+}