Do not link with libnotify

[zcbenz]

We are already loading it dynamically there is no meaning linking with the library anymore.

Close electron/electron#5348.

[anaisbetts]

This seems to break debug build:

.a /home/paul/code/electron/electron/vendor/brightray/vendor/download/libchromiumcontent/shared_library/libcdm_renderer.a /home/paul/code/electron/electron/vendor/brightray/vendor/download/libchromiumcontent/shared_library/libsecurity_state.a -ldl
obj/vendor/brightray/browser/linux/brightray.libnotify_loader.o: In function `LibNotifyLoader::Load(std::string const&)':
../../vendor/brightray/browser/linux/libnotify_loader.cc:(.text+0x186): undefined reference to `notify_is_initted'
../../vendor/brightray/browser/linux/libnotify_loader.cc:(.text+0x1d2): undefined reference to `notify_init'
../../vendor/brightray/browser/linux/libnotify_loader.cc:(.text+0x221): undefined reference to `notify_get_server_info'
../../vendor/brightray/browser/linux/libnotify_loader.cc:(.text+0x270): undefined reference to `notify_get_server_caps'
../../vendor/brightray/browser/linux/libnotify_loader.cc:(.text+0x2bf): undefined reference to `notify_notification_new'
../../vendor/brightray/browser/linux/libnotify_loader.cc:(.text+0x30e): undefined reference to `notify_notification_add_action'
../../vendor/brightray/browser/linux/libnotify_loader.cc:(.text+0x35d): undefined reference to `notify_notification_set_image_from_pixbuf'
../../vendor/brightray/browser/linux/libnotify_loader.cc:(.text+0x3ac): undefined reference to `notify_notification_set_timeout'
../../vendor/brightray/browser/linux/libnotify_loader.cc:(.text+0x3fb): undefined reference to `notify_notification_set_hint_string'
../../vendor/brightray/browser/linux/libnotify_loader.cc:(.text+0x44a): undefined reference to `notify_notification_show'
../../vendor/brightray/browser/linux/libnotify_loader.cc:(.text+0x499): undefined reference to `notify_notification_close'