Add portlock directory to .gitignore

Commit 9b4eafc added creattion of a directory to reserve TAP test
ports at the top of the build tree. In a non-vpath build this means at
the top of the source tree, so it needs to be added to .gitignore.

As suggested by Michael Paquier

Backpatch to all live branches.

Author: adunstan

.gitignore

@@ -42,3 +42,4 @@ lib*.pc
 /Debug/
 /Release/
 /tmp_install/
+/portlock/