Don't track this library generated directory.

diff --git a/.gitignore b/.gitignore
index 839bd3f..a4e7cb4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,3 +2,6 @@
 *.a
 *.so
 *.mod
+
+# Generated directory that doesn't need to be tracked:
+lib/Lbfgsb.3.0