syscalls: Adjust /proc/net/dev parser for old kernels.

* guix/build/syscalls.scm (%interface-line): Remove whitespace in rest
  pattern.  (There's no extra whitespace after the colon with Linux
  2.6.32.)

guix/build/syscalls.scm

@@ -295,7 +295,7 @@ (define* (network-interfaces #:optional sock)
 
 (define %interface-line
   ;; Regexp matching an interface line in Linux's /proc/net/dev.
-  (make-regexp "^[[:blank:]]*([[:alnum:]]+): .*$"))
+  (make-regexp "^[[:blank:]]*([[:alnum:]]+):.*$"))
 
 (define (all-network-interfaces)
   "Return all the registered network interfaces, including those that are not