.gitignore: only ignore cockpit.socket

We only dynamically generate a single .socket file now.  The rest of
them are in version control and installed verbatim.

.gitignore

@@ -76,7 +76,7 @@ Makefile.in
 /src/cockpit.egg-info/
 /src/common/fail-html.c
 /src/systemd/cockpit*.service
-/src/systemd/cockpit*.socket
+/src/systemd/cockpit.socket
 /src/systemd/tmpfiles.d/cockpit-ws.conf
 /src/tls/cockpit-certificate-helper
 /src/ws/cockpit-desktop