cmd: zip: fix implicit declaration warning

Fix the following warning:

cmd/zip.c: In function ‘do_zip’:
cmd/zip.c:30:6: warning: implicit declaration of function ‘gzip’; did you mean ‘do_zip’? [-Wimplicit-function-declaration]
  if (gzip((void *) dst, &dst_len, (void *) src, src_len) != 0)
      ^~~~
      do_zip

Include gzip.h header which declares the gzip() function.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Simon Glass <sjg@chromium.org>