PostgreSQL 19 support

[wolfgangwalther]

Currently fails to build with PostgreSQL 19 beta 1. Tail of the log:

      |   ^~~~~~
pc_access.c:249:3: error: use of undeclared identifier 'bitmap'
  249 |   bitmap = ARR_NULLBITMAP(array);
      |   ^~~~~~
pc_access.c:253:27: error: use of undeclared identifier 'bitmap'
  253 |     if (!array_get_isnull(bitmap, i))
      |                           ^~~~~~
pc_access.c:1130:3: error: use of undeclared identifier 'bits8'
 1130 |   bits8 *bitmap = ARR_NULLBITMAP(array);
      |   ^~~~~
pc_access.c:1130:10: error: use of undeclared identifier 'bitmap'
 1130 |   bits8 *bitmap = ARR_NULLBITMAP(array);
      |          ^~~~~~
pc_access.c:1134:26: error: use of undeclared identifier 'bitmap'
 1134 |     if (array_get_isnull(bitmap, i))
      |                          ^~~~~~
14 errors generated.
make[1]: *** [<builtin>: pc_access.o] Error 1
make[1]: Leaving directory '/build/source/pgsql'
make: *** [Makefile:4: all] Error 2