gitignore: add libtool

Add libtool artifact to gitignore.

Change-Id: Ie283f97cc0d714be89ebed420f439322cda653b1
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/.gitignore b/.gitignore
index 1872c14..27ec686 100644
--- a/.gitignore
+++ b/.gitignore
@@ -27,3 +27,5 @@
 *.sw*

 *.log

 *.trs

+libtool

+