meson: fix undefined function in meson_post_install.py

join_paths() is a function defined in meson. Use os.path.join() instead.

(cherry picked from commit 59d2bf7)

meson_post_install.py

@@ -3,10 +3,13 @@
 import os
 import subprocess
 
-schemadir = os.path.join(os.environ['MESON_INSTALL_PREFIX'], 'share', 'glib-2.0', 'schemas')
+install_prefix = os.environ['MESON_INSTALL_PREFIX']
+icondir = os.path.join(install_prefix, 'share', 'icons', 'hicolor')
+schemadir = os.path.join(install_prefix, 'share', 'glib-2.0', 'schemas')
 
 if not os.environ.get('DESTDIR'):
-    print('Compiling gsettings schemas...')
-    subprocess.call(['glib-compile-schemas', join_paths(gl_datadir, 'glib-2.0', 'schemas')])
     print('Updating icon cache...')
-    subprocess.call(['gtk-update-icon-cache --ignore-theme-index --force', join_paths(gl_datadir, 'icons', 'hicolor')])
+    subprocess.call(['gtk-update-icon-cache', '--ignore-theme-index', '--force', icondir])
+
+    print('Compiling gsettings schemas...')
+    subprocess.call(['glib-compile-schemas', schemadir])