Fix minor GCC warning on Win32 backends #1163

Job	Run time
Emscripten	4m 48s
Linux (clang, clang++, -DENABLE_PRECOMPILED_HEADERS=OFF -DSAMPLES_BACKEND=GLFW_GL2)	2m 26s
Linux (clang, clang++, -DSAMPLES_BACKEND=SDL_VK)	1m 56s
Linux (-DBUILD_TESTING=ON -DENABLE_PRECOMPILED_HEADERS=OFF, true)	3m 30s
Linux (-DNO_FONT_INTERFACE_DEFAULT=ON -DENABLE_LOTTIE_PLUGIN=ON -DSAMPLES_BACKEND=X11_GL2)	2m 15s
Linux (-DDISABLE_RTTI_AND_EXCEPTIONS=ON -DSAMPLES_BACKEND=SDL_GL2)	2m 10s
Linux (-DNO_THIRDPARTY_CONTAINERS=ON -DSAMPLES_BACKEND=SFML_GL2)	2m 24s
Linux (-DSAMPLES_BACKEND=SDL_VK -DRMLUI_VK_DEBUG=ON -DENABLE_PRECOMPILED_HEADERS=OFF -DCMAKE_BUIL...	2m 43s
macOS (-DSAMPLES_BACKEND=auto)	5m 26s
macOS (-DSAMPLES_BACKEND=GLFW_GL2)	7m 10s
Windows (-DSAMPLES_BACKEND=auto -DENABLE_PRECOMPILED_HEADERS=OFF)	10m 36s
Windows (-DSAMPLES_BACKEND=Win32_VK -DRMLUI_VK_DEBUG=ON)	7m 23s
Windows (-DSAMPLES_BACKEND=SDL_VK -DBUILD_LUA_BINDINGS_FOR_LUAJIT=ON)	7m 37s
	1h 0m 24s

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