Fix build error on OpenWrt build servers. See:

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

diff -u --recursive grilo-plugins-0.3.10-vanilla/meson.build grilo-plugins-0.3.10/meson.build
--- grilo-plugins-0.3.10-vanilla/meson.build	2019-09-12 11:48:29.000000000 -0400
+++ grilo-plugins-0.3.10/meson.build	2020-01-20 11:17:22.803149935 -0500
@@ -173,6 +173,3 @@
 rootdir = include_directories('.')
 
 subdir('src')
-subdir('help')
-subdir('po')
-subdir('tests')