Add attention request function for each platform

Add stub functions ```_glfwPlatformRequestWindowAttention``` for the
platforms Cocoa, Mir, and Wayland, which still lack the implementation
of attention request.

src/cocoa_window.m

@@ -1272,6 +1272,10 @@ void _glfwPlatformShowWindow(_GLFWwindow* window)
     [window->ns.object orderFront:nil];
 }
 
+void _glfwPlatformRequestWindowAttention(_GLFWwindow* window)
+{
+}
+
 void _glfwPlatformHideWindow(_GLFWwindow* window)
 {
     [window->ns.object orderOut:nil];

src/mir_window.c

@@ -567,6 +567,10 @@ void _glfwPlatformShowWindow(_GLFWwindow* window)
     mir_surface_spec_release(spec);
 }
 
+void _glfwPlatformRequestWindowAttention(_GLFWwindow* window)
+{
+}
+
 void _glfwPlatformFocusWindow(_GLFWwindow* window)
 {
     _glfwInputError(GLFW_PLATFORM_ERROR,

src/wl_window.c

@@ -589,6 +589,10 @@ void _glfwPlatformShowWindow(_GLFWwindow* window)
     }
 }
 
+void _glfwPlatformRequestWindowAttention(_GLFWwindow* window)
+{
+}
+
 void _glfwPlatformHideWindow(_GLFWwindow* window)
 {
     if (!window->monitor)