neighbor/meson.build: disable if -Ddatabase=false

Fixes yet another build failure (which however only affects the 0.22
branch).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/666

src/neighbor/meson.build

@@ -1,4 +1,4 @@
-if not get_option('neighbor')
+if not get_option('neighbor') or not enable_database
   conf.set('ENABLE_NEIGHBOR_PLUGINS', false)
   neighbor_glue_dep = dependency('', required: false)
   subdir_done()