Ignore buildtest artifacts.

Reviewed-by: Richard Levitte <levitte@openssl.org>

.gitignore

diff --git a/.gitignore b/.gitignore
index 508fe0b..ec77c32 100644
--- a/.gitignore
+++ b/.gitignore
@@ -31,6 +31,7 @@
 /test/test*.pem
 /test/newkey.pem
 /test/*.log
+/test/buildtest_*
 
 # Certificate symbolic links
 *.0