docs: add XML util to python requirements

Signed-off-by: Alessandro Comodi <acomodi@antmicro.com>

docs/requirements.txt

diff --git a/docs/requirements.txt b/docs/requirements.txt
index 676c8ee..accf455 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -21,3 +21,6 @@
 
 # V2X
 -e ../
+
+# XML utils
+git+https://github.com/SymbiFlow/vtr-xml-utils.git#egg=vtr-xml-utils