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
1 file changed