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

Commit 782620f 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: #9077
Reviewed-By: Ben Noordhuis <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Myles Borins <[email protected]>
Reviewed-By: Johan Bergström <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>
Reviewed-By: Evan Lucas <[email protected]>

node.gyp

@@ -480,6 +480,8 @@
         }],
         [ 'node_shared_zlib=="false"', {
           'dependencies': [ 'deps/zlib/zlib.gyp:zlib' ],
+        }, {
+          'defines': [ 'ZLIB_CONST' ],
         }],
 
         [ 'node_shared_http_parser=="false"', {