build: file-systems: Import (guix build syscalls) for non-static Guiles.

* gnu/build/file-systems.scm: Import (guix build syscalls) when 'mount' is not
  defined.
* gnu/system.scm (operating-system-activation-script): Include (guix build
  syscalls) module in derivation.

gnu/build/file-systems.scm

@@ -50,6 +50,13 @@ (define-module (gnu build file-systems)
 ;;;
 ;;; Code:
 
+;; 'mount' is already defined in the statically linked Guile used for initial
+;; RAM disks, but in all other cases the (guix build syscalls) module contains
+;; the mount binding.
+(unless (defined? 'mount)
+  (module-use! (current-module)
+               (resolve-interface '(guix build syscalls))))
+
 ;; Linux mount flags, from libc's <sys/mount.h>.
 (define MS_RDONLY 1)
 (define MS_NOSUID 2)

gnu/system.scm

@@ -689,6 +689,7 @@ (define %modules
       (gnu build linux-modules)
       (gnu build file-systems)
       (guix build utils)
+      (guix build syscalls)
       (guix elf)))
 
   (define (service-activations services)