[gssdp] sniffer: Fix building as subproject

[Jens Georg <jensgeorg src gnome org>]

commit 0ddf2fe41ca03593a7df5983bb423f3c1ab71d78
Author: Jens Georg <mail jensge org>
Date:   Sat Aug 14 10:04:36 2021 +0200

    sniffer: Fix building as subproject

 tools/meson.build | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/tools/meson.build b/tools/meson.build
index 3de758c..aae3255 100644
--- a/tools/meson.build
+++ b/tools/meson.build
@@ -1,7 +1,7 @@
 resource = gnome.compile_resources(
     'org.gupnp.GSSDP.DeviceSniffer',
     'gssdp-device-sniffer.gresource.xml',
-    source_dir: [meson.current_source_dir(), meson.source_root() / 'doc']
+    source_dir: [meson.current_source_dir(), meson.current_source_dir() / .. / 'doc']
 )
 
 sniffer = executable(