build: add -DZLIB_CONST when building with --shared-zlib

Commit 782620f0 added the define only when building with the bundled
zlib. Using a shared zlib results in build breakage:

../src/inspector_agent.cc:179:16: error: assigning to 'Bytef *' (aka 'unsigned char *') from incompatible type
      'const uint8_t *' (aka 'const unsigned char *')
  strm.next_in = PROTOCOL_JSON + 3;
               ^ ~~~~~~~~~~~~~~~~~
1 error generated.

PR-URL: https://github.com/nodejs/node/pull/9077


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>