build: ignore mkinstalldirs and texinfo.tex

these are autogenerated by autoreconf, drop them from git.
diff --git a/doc/.gitignore b/doc/.gitignore
index ed527fc..5071f98 100644
--- a/doc/.gitignore
+++ b/doc/.gitignore
@@ -6,6 +6,7 @@
 zebra.html
 defines.texi
 version.texi
+texinfo.tex
 quagga.html
 quagga.info
 *.pdf