build: ignore mkinstalldirs and texinfo.tex

these are autogenerated by autoreconf, drop them from git.
diff --git a/.gitignore b/.gitignore
index 61ee8e6..516ed61 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,6 +15,7 @@
 depcomp
 missing
 install-sh
+mkinstalldirs
 autom4te*.cache
 configure.lineno
 configure