etc: committer: Support reading G-expressions.

Some package definitions use G-expressions (see, e.g., chez-scheme).
Import (guix gexp) such that Guile knows how to read those.
Otherwise, an exception such as the following might be raised:

ERROR: In procedure read:
In procedure scm_lreadr: gnu/services/networking.scm:480:16: Unknown # object: #\~

* etc/committer.scm.in: Import (guix gexp).

Signed-off-by: Ricardo Wurmus <rekado@elephly.net>

etc/committer.scm.in

@@ -36,7 +36,8 @@
         (ice-9 popen)
         (ice-9 match)
         (ice-9 rdelim)
-        (ice-9 textual-ports))
+        (ice-9 textual-ports)
+        (guix gexp))
 
 (define* (break-string str #:optional (max-line-length 70))
   "Break the string STR into lines that are no longer than MAX-LINE-LENGTH.