video_core: Enable sign conversion warnings

Enable sign conversion warnings but don't treat them as errors.

src/video_core/CMakeLists.txt

@@ -184,5 +184,5 @@ endif()
 if (MSVC)
     target_compile_options(video_core PRIVATE /we4267)
 else()
-    target_compile_options(video_core PRIVATE -Werror=conversion -Wno-sign-conversion)
+    target_compile_options(video_core PRIVATE -Werror=conversion -Wno-error=sign-conversion)
 endif()