Fix build error on OpenWrt build servers. See:

	https://github.com/openwrt/packages/pull/10433#issuecomment-549242840
	https://github.com/mesonbuild/meson/issues/821

--- a/meson.build
+++ b/meson.build
@@ -192,6 +192,3 @@ gnome = import('gnome')
 rootdir = include_directories('.')
 
 subdir('src')
-subdir('help')
-subdir('po')
-subdir('tests')