Backends: Win32: Fixed warning with old MinGW/GCC versions.

backends/imgui_impl_win32.cpp

@@ -98,6 +98,7 @@ typedef DWORD(WINAPI* PFN_XInputGetState)(DWORD, XINPUT_STATE*);
 #endif
 #if defined(__GNUC__)
 #pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpragmas"                  // warning: unknown option after '#pragma GCC diagnostic' kind
 #pragma GCC diagnostic ignored "-Wcast-function-type"       // warning: cast between incompatible function types (for loader)
 #endif