bug#25409: make fails when guile-json is not present

[Ben Woodcroft <b.woodcroft@uq.edu.au>]

Hi,

Since "#:use-module (guix docker)" is at the top of 
guix/scripts/refresh.scm and "#:use-module (json)" is at the top of 
guix/docker.scm, when the json module is unavailable make fails:

[...]

In unknown file:
    ?: 9 [primitive-load-path "guix/docker" ...]
In ice-9/eval.scm:
  505: 8 [#<procedure 561cba494500 at ice-9/eval.scm:499:4 (exp)> #]
In ice-9/psyntax.scm:
1106: 7 [expand-top-sequence ((define-module # # # ...)) () ((top)) ...]
  989: 6 [scan ((define-module (guix docker) #:use-module ...)) () ...]
  279: 5 [scan ((#(syntax-object let # ...) (#) (# #) ...)) () ...]
In ice-9/eval.scm:
  411: 4 [eval # ()]
In ice-9/boot-9.scm:
2951: 3 [define-module* (guix docker) #:filename ...]
2926: 2 [resolve-imports (((guix hash)) ((guix store)) ((guix utils)) ...)]
2867: 1 [resolve-interface (json) #:select ...]
In unknown file:
    ?: 0 [scm-error misc-error #f "~A ~S" ("no code for module" (json)) #f]

[...]

I'm under the impression that guile-json is still an optional dependency.
Thanks, ben