tests/server/.gitignore: Add socksd to the ignore list

Missed in 04fd6755.

Closes #3978
diff --git a/tests/server/.gitignore b/tests/server/.gitignore
index 497783b..d410f5e 100644
--- a/tests/server/.gitignore
+++ b/tests/server/.gitignore
@@ -5,3 +5,4 @@
 sockfilt
 sws
 tftpd
+socksd