Silence warnings from libnanomsg on Mac like we do on Linux.

This will silence these warnings:

../../third_party/externals/nanomsg/src/core/global.c:162:34: warning: missing field 'unused' initializer [-Wmissing-field-initializers]
static struct nn_global self = {0};

../../third_party/externals/nanomsg/src/utils/clock.c:44:61: warning: missing field 'denom' initializer [-Wmissing-field-initializers]
static mach_timebase_info_data_t nn_clock_timebase_info = {0};

BUG=skia:

Review URL: https://codereview.chromium.org/803113003

gyp/nanomsg.gyp

@@ -167,6 +167,9 @@
         ],
       }],
       ['skia_os == "mac"', {
+        'xcode_settings': {
+            'WARNING_CFLAGS': [ '-w' ],
+        },
         'defines=': [             # equals sign throws away most Skia defines (just noise)
           'HAVE_ARPA_INET_H',
           'HAVE_DLFCN_H',