meson: Support building builtin without seatd

The builtin backend relies on the seatd backend implementation. When
builtin was enabled without seatd, compilation would fail due to the
implementation not being included.

Include the implementation if either seatd or builtin is enabled.

meson.build

@@ -106,8 +106,11 @@ with_seatd = get_option('seatd') == 'enabled'
 with_builtin = get_option('builtin') == 'enabled'
 with_server = get_option('server') == 'enabled'
 
-if with_seatd
+if with_seatd or with_builtin
 	private_files += 'libseat/backend/seatd.c'
+endif
+
+if with_seatd
 	add_project_arguments('-DSEATD_ENABLED=1', language: 'c')
 endif