Fix meson warning due to missing declaration of NM

Fix meson warning due to missing declaration of NM

The warning was showing up in the early stages of the meson build, when
the contents of Makefile.global is generated based on the configuration
of meson for PGXS.

NM is added to pgxs_empty.  This declaration is only used internally for
the libpq sanity check, so there is no point in exposing it in PGXS.

Oversight in 4a8e6f43a6b5.

Reported-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/4423e01f-1e52-4f47-a6ca-05cc8081c888@eisentraut.org