gnu: lua5.1-socket: Add missing file-name field.

* gnu/packages/lua.scm (lua5.1-socket)[source]: Add missing file-name
field.

gnu/packages/lua.scm

@@ -179,6 +179,7 @@ (define-public lua5.1-socket
               (uri (string-append
                     "https://github.com/diegonehab/luasocket/archive/v"
                     version ".tar.gz"))
+              (file-name (string-append name "-" version ".tar.gz"))
               (sha256
                (base32
                 "0j8jx8bjicvp9khs26xjya8c495wrpb7parxfnabdqa5nnsxjrwb"))))