lnd: use single asterisk for gitignore wildcards

This stops ripgrep from complaining about

  invalid use of **; must be one path component

See https://github.com/BurntSushi/ripgrep/issues/373

.gitignore

@@ -30,13 +30,13 @@ lncli
 output*.log
 
 cmd/cmd
-**.key
+*.key
-**.hex
+*.hex
 
 cmd/lncli/lncli
 
 # vim
-**.swp
+*.swp
 
 *.hex
 *.db