Specify a path to the ITS rules

Use our own ITS rules found under data/its. This resolves a
configuration breakage on some systems:

msgfmt: cannot locate ITS rules for data/org.freedesktop.ModemManager1.policy.in

According to the msgfmt manual, we need to specify a path to the parent
directory containing its folder. Hence, we add a path to data as the
data_dirs option.

data/meson.build

@@ -58,6 +58,7 @@ if enable_polkit
     ),
     output: '@BASENAME@',
     po_dir: po_dir,
+    data_dirs: [data_dir],
     install: true,
     install_dir: polkit_gobject_policydir,
   )

meson.build

@@ -63,6 +63,7 @@ build_root = meson.current_build_dir()
 build_aux_dir = source_root / 'build-aux'
 templates_dir = source_root / 'build-aux/templates'
 po_dir = source_root / 'po'
+data_dir = source_root / 'data'
 src_dir = source_root / 'src'
 plugins_dir = source_root / 'src/plugins'