meson: Replace join_path with intuitive slash (#213)

Author: ryonakano

platform-data/data/meson.build

@@ -1,17 +1,17 @@
 i18n.merge_file(
     input: 'platform.appdata.xml.in',
     output: 'io.elementary.Platform.appdata.xml',
-    po_dir: join_paths(meson.project_source_root(), 'po'),
+    po_dir: meson.project_source_root() / 'po',
     type: 'xml',
     install: true,
-    install_dir: join_paths(get_option('datadir'), 'metainfo'),
+    install_dir: get_option('datadir') / 'metainfo',
 )
 
 i18n.merge_file(
     input: 'sdk.appdata.xml.in',
     output: 'io.elementary.Sdk.appdata.xml',
-    po_dir: join_paths(meson.project_source_root(), 'po'),
+    po_dir: meson.project_source_root() / 'po',
     type: 'xml',
     install: true,
-    install_dir: join_paths(get_option('datadir'), 'metainfo'),
+    install_dir: get_option('datadir') / 'metainfo',
 )