build: use meson.override_dependency

This allows downstream users to write:

    dependency('libseat', fallback: 'seatd')

instead of having to rely on the seatd/meson.build's variable name:

    dependency('libseat', fallback: ['seatd', 'libseat'])

meson.build

@@ -200,6 +200,8 @@ libseat = declare_dependency(
 	variables: libseat_vars,
 )
 
+meson.override_dependency('libseat', libseat)
+
 if with_server
 	executable(
 		'seatd',