Backends: Fixed pragma warning with old GCC.

backends/imgui_impl_opengl3.cpp

@@ -112,6 +112,7 @@
 #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 "-Wunknown-warning-option"   // warning: unknown warning group 'xxx'
 #pragma GCC diagnostic ignored "-Wcast-function-type"       // warning: cast between incompatible function types
 #endif