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/CMakeLists.txt

@@ -48,7 +48,8 @@ elseif (_GLFW_OSMESA)
     target_sources(glfw PRIVATE null_platform.h null_joystick.h posix_time.h
                                 posix_thread.h osmesa_context.h null_init.c
                                 null_monitor.c null_window.c null_joystick.c
-                                posix_time.c posix_thread.c osmesa_context.c)
+                                posix_time.c posix_thread.c egl_context.c
+                                osmesa_context.c)
 endif()
 
 if (_GLFW_X11 OR _GLFW_WAYLAND)