gitignore: Ignore 'doc' files generated at runtime

The list is based on doc/Makefile 'clean' target
Signed-off-by: Vadim Kochan <vadim4j@gmail.com>

.gitignore

@@ -35,3 +35,12 @@ series
 # tests
 testsuite/results
 testsuite/iproute2/iproute2-this
+
+# doc files generated at runtime
+doc/*.aux
+doc/*.log
+doc/*.toc
+doc/*.ps
+doc/*.dvi
+doc/*.html
+doc/*.pdf