Backends: GLFW: Add define guards for glfw native defines. (#7536)

backends/imgui_impl_glfw.cpp

@@ -85,11 +85,15 @@
 
 #ifdef _WIN32
 #undef APIENTRY
+#ifndef GLFW_EXPOSE_NATIVE_WIN32
 #define GLFW_EXPOSE_NATIVE_WIN32
+#endif
 #include <GLFW/glfw3native.h>   // for glfwGetWin32Window()
 #endif
 #ifdef __APPLE__
+#ifndef GLFW_EXPOSE_NATIVE_COCOA
 #define GLFW_EXPOSE_NATIVE_COCOA
+#endif
 #include <GLFW/glfw3native.h>   // for glfwGetCocoaWindow()
 #endif