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

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

build

Description of change

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.