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/store.scm: Use (ice-9 threads).

guix/store.scm

@@ -40,6 +40,7 @@ (define-module (guix store)
   #:use-module (ice-9 regex)
   #:use-module (ice-9 vlist)
   #:use-module (ice-9 popen)
+  #:use-module (ice-9 threads)
   #:use-module (web uri)
   #:export (%daemon-socket-uri
             %gc-roots-directory