guix: Fix Guile current-processor-count deprecation warnings.

When current-processor-count is used without (ice-9 threads) being used, Guile
complains with the following warning:

Import (ice-9 threads) to have access to `current-processor-count'.

* guix/build/utils.scm: Use (ice-9 threads).

guix/build/utils.scm

@@ -29,6 +29,7 @@
   #:use-module (ice-9 regex)
   #:use-module (ice-9 rdelim)
   #:use-module (ice-9 format)
+  #:use-module (ice-9 threads)
   #:use-module (rnrs bytevectors)
   #:use-module (rnrs io ports)
   #:re-export (alist-cons