build: go-build-system: Follow-up commit.

There was an extraneous pair of parens in commit 7e84d3eef7.  Thanks for Mark
Weaver for reporting the issue.

* guix/build/go-build-system.scm (unpack): Remove the extraneous pair of
  parentheses surrounding the `display' function call.

guix/build/go-build-system.scm

@@ -179,7 +179,7 @@ unpacking."
       (delete-file-recursively scratch-dir)))
 
   (when (string-null? import-path)
-    ((display "WARNING: The Go import path is unset.\n")))
+    (display "WARNING: The Go import path is unset.\n"))
   (when (string-null? unpack-path)
     (set! unpack-path import-path))
   (let ((dest (string-append (getenv "GOPATH") "/src/" unpack-path)))