l10n: Correct GETTEXT_PACKAGE (#399)

Author: ryonakano

meson.build

@@ -24,7 +24,7 @@ add_project_arguments(
 
 config_data = configuration_data()
 config_data.set_quoted('LOCALEDIR', join_paths(get_option('prefix'), get_option('localedir')))
-config_data.set_quoted('GETTEXT_PACKAGE', meson.project_name() + '-plug')
+config_data.set_quoted('GETTEXT_PACKAGE', gettext_name)
 config_file = configure_file(
     input: 'src/Config.vala.in',
     output: '@BASENAME@',