Backends: OpenGL3: Fix for zealous warning. (#5947)

Consider moving to imgui_impl_opengl3_loader.h ?

backends/imgui_impl_opengl3.cpp

@@ -110,6 +110,10 @@
 #pragma clang diagnostic ignored "-Wold-style-cast"     // warning: use of old-style cast
 #pragma clang diagnostic ignored "-Wsign-conversion"    // warning: implicit conversion changes signedness
 #endif
+#if defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wcast-function-type"   // warning: cast between incompatible function types
+#endif
 
 // GL includes
 #if defined(IMGUI_IMPL_OPENGL_ES2)
@@ -885,6 +889,9 @@ void    ImGui_ImplOpenGL3_DestroyDeviceObjects()
     ImGui_ImplOpenGL3_DestroyFontsTexture();
 }
 
+#if defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
 #if defined(__clang__)
 #pragma clang diagnostic pop
 #endif