Win32: Fix invalid preprocessor command 'warning'

The GCCism #warning had snuck into the Win32 platform code.

Merges #1953.

src/win32_init.c

@@ -40,7 +40,7 @@ static const GUID _glfw_GUID_DEVINTERFACE_HID =
 #if defined(_GLFW_USE_HYBRID_HPG) || defined(_GLFW_USE_OPTIMUS_HPG)
 
 #if defined(_GLFW_BUILD_DLL)
- #warning "These symbols must be exported by the executable and have no effect in a DLL"
+ #pragma message("These symbols must be exported by the executable and have no effect in a DLL")
 #endif
 
 // Executables (but not DLLs) exporting this symbol with this value will be