deps: suppress zlib compiler warnings

Currently, there are a number of compilation warnings from zlib like the
following one:

../deps/zlib/infback.c: In function ‘inflateBack’:
../deps/zlib/infback.c:479:25: warning:
this statement may fall through [-Wimplicit-fallthrough=]
  479 |             state->mode = LEN;
      |             ~~~~~~~~~~~~^~~~~
../deps/zlib/infback.c:481:9: note: here
  481 |         case LEN:
      |         ^~~~

In this case there is no break statement and the intention is to fall
through:

           Tracev((stderr, "inflate:       codes ok\n"));
           state->mode = LEN;

        case LEN:

This commit adds -Wno-implicit-fallthrough' to zlib.gyp to suppress
these warnings.

deps/zlib/zlib.gyp

@@ -51,6 +51,7 @@
               '.',
             ],
           },
+          'cflags': [ '-Wno-implicit-fallthrough' ],
           'conditions': [
             ['OS!="win"', {
               'cflags!': [ '-ansi' ],