Null: Add non-functional EGL support

This is done in preparation for runtime platform selection, to make sure
every platform can build with EGL enabled.

It may be possible to add support for things like the ANGLE null
platform later.

src/null_init.c

@@ -48,10 +48,11 @@ void _glfwPlatformTerminate(void)
 {
     _glfw_free(_glfw.null.clipboardString);
     _glfwTerminateOSMesa();
+    _glfwTerminateEGL();
 }
 
 const char* _glfwPlatformGetVersionString(void)
 {
-    return _GLFW_VERSION_NUMBER " null OSMesa";
+    return _GLFW_VERSION_NUMBER " null OSMesa EGL";
 }