Add -Wmissing-variable-declarations to common warnings

src/catch2/internal/catch_main.cpp

@@ -14,7 +14,7 @@
 namespace Catch {
     CATCH_INTERNAL_START_WARNINGS_SUPPRESSION
     CATCH_INTERNAL_SUPPRESS_GLOBALS_WARNINGS
-    LeakDetector leakDetector;
+    static LeakDetector leakDetector;
     CATCH_INTERNAL_STOP_WARNINGS_SUPPRESSION
 }