Examples: Add missing include guards (#2166)

examples/imgui_impl_glfw.h

@@ -16,6 +16,8 @@
 // The 'glsl_version' initialization parameter defaults to "#version 150" if NULL.
 // Only override if your GL version doesn't handle this GLSL version. Keep NULL if unsure!
 
+#pragma once
+
 struct GLFWwindow;
 
 IMGUI_IMPL_API bool     ImGui_ImplGlfw_InitForOpenGL(GLFWwindow* window, bool install_callbacks);