build: Make it possible to use grilo-plugins as a Meson subproject

This commit adds a dummy declare_dependency() object. This is needed
so that other projects can embed grilo-plugins as a Meson subproject,
like this:

    dependency('grilo-plugins-0.3', version: '>= 0.3.9',
               fallback: ['grilo-plugins', 'grilo_plugins_dep']
1 job for master in 4 minutes and 51 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed build_stable #1227280

00:04:51