gnu: smalltalk: Move 'zip' from 'inputs' to 'native-inputs'.

The 'zip' package is not in the references of 'smalltalk'.
As such, it probably should be a native input, not a regular
input. This can be confirmed by trying to cross-compile
the 'smalltalk' package, without this commit. The configure
script will ask to install InfoZIP.

* gnu/packages/smalltalk.scm
  (smalltalk)[inputs]{zip}: Move to ...
  (smalltalk)[native-inputs]{zip} ... here.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>

gnu/packages/smalltalk.scm

@@ -64,7 +64,8 @@
        ;; XXX: To be removed with the next release of Smalltalk.
        ("autoconf" ,autoconf)
        ("automake" ,automake)
-       ("libtool" ,libtool)))
+       ("libtool" ,libtool)
+       ("zip" ,zip)))
     ;; TODO: These optional dependencies raise the closure size to ~1 GiB
     ;; from the current ~100 MiB, although some of them might be very
     ;; useful for end users:
@@ -81,8 +82,7 @@
        ("libffi" ,libffi)
        ("libltdl" ,libltdl)
        ("libsigsegv" ,libsigsegv)
-       ("lightning" ,lightning)
-       ("zip" ,zip)))
+       ("lightning" ,lightning)))
     (arguments
      `(#:phases
        (modify-phases %standard-phases