gitignore: Add missing libtool

libtool is a generated script that is missing from the current gitignore
list.

Change-Id: I94324417c2bff9a0ac34f8602f22d322a6147827
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/.gitignore b/.gitignore
index 77b0c3e..abf6d99 100644
--- a/.gitignore
+++ b/.gitignore
@@ -22,6 +22,7 @@
 depcomp
 install-sh
 i586-openbmc-linux-libtool
+libtool
 ltmain.sh
 missing
 stamp-h1