CI: Fix macOS build

Author: joto

.github/workflows/c.yml

@@ -57,7 +57,7 @@ jobs:
     env:
       CC: clang
       CXX: clang++
-      CXXFLAGS: -Werror -Wall -pedantic
+      CXXFLAGS: -Werror -Wall -pedantic -Wno-nullability-extension
       BUILD_TYPE: Debug
     steps:
       - uses: actions/checkout@v4