doc: Include doxmlparser on requirements.txt

It will be necessary to parse the doxygen XML files to extract `since`
and `version` from them.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>
diff --git a/doc/requirements.txt b/doc/requirements.txt
index b1f20fb..a105501 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -16,3 +16,6 @@
 
 # Used by pytest-twister-harness plugin
 pytest
+
+# Doxygen doxmlparser
+doxmlparser