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 4a8e6f43a6.

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

src/makefiles/meson.build

@@ -154,9 +154,10 @@ pgxs_bins = {
 pgxs_empty = [
   'ICU_CFLAGS', # needs to be added, included by public server headers
 
-  # hard to see why we'd need either?
+  # hard to see why we'd need these ones?
   'ZIC',
   'TCLSH',
+  'NM',
 
   # docs don't seem to be supported by pgxs
   'XMLLINT',