Silence meson warning about PG_TEST_EXTRA in src/Makefile.global.in

Commit 99b937a44f introduced this warning when you run "meson setup":

    Configuring Makefile.global using configuration
    ../src/meson.build:31: WARNING: The variable(s) 'PG_TEST_EXTRA' in the input file 'src/Makefile.global.in' are not present in the given configuration data.

To fix, add PG_TEST_EXTRA to the list of variables that are not needed
in the makefiles generated by meson. In meson builds, the makefiles
are only used for PGXS, not for building or testing the server itself.

Reported-by: Peter Eisentraut
Discussion: https://www.postgresql.org/message-id/5c380997-e270-425a-9542-e4ef36a285de@eisentraut.org

src/makefiles/meson.build

@@ -182,6 +182,7 @@ pgxs_empty = [
 
   # Not needed because we don't build the server / PLs with the generated makefile
   'LIBOBJS', 'PG_CRC32C_OBJS', 'PG_POPCNT_OBJS', 'TAS',
+  'PG_TEST_EXTRA',
   'DTRACEFLAGS', # only server has dtrace probes
 
   'perl_archlibexp', 'perl_embed_ccflags', 'perl_embed_ldflags', 'perl_includespec', 'perl_privlibexp',