unix: define _GNU_SOURCE, exposes glibc-isms
unix: define _GNU_SOURCE, exposes glibc-isms

EAI_NODATA and some other getaddrinfo() error codes are returned by
glibc but not exposed in the headers unless _GNU_SOURCE is defined.

Only define in src/uv-common.c because that's the only file that deals
with EAI_* error codes.

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