Ignore dirty libxdp submodule

Make Git ignore untracked files in libxdp submodule.

Signed-off-by: Simon Sundberg <simon.sundberg@kau.se>

Author: simosund

.gitmodules

@@ -5,3 +5,4 @@
 [submodule "lib/xdp-tools"]
 	path = lib/xdp-tools
 	url = https://github.com/xdp-project/xdp-tools
+	ignore = dirty