PostgreSQL 19 support

[wolfgangwalther]

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

hamming.c:47:10: error: use of undeclared identifier 'pa'
   47 |         bits8           *pa, *pb;
      |                          ^~
hamming.c:47:15: error: use of undeclared identifier 'pb'
   47 |         bits8           *pa, *pb;
      |                               ^~
hamming.c:69:2: error: use of undeclared identifier 'pa'
   69 |         pa = VARBITS(a);
      |         ^~
hamming.c:70:2: error: use of undeclared identifier 'pb'
   70 |         pb = VARBITS(b);
      |         ^~
hamming.c:74:8: error: use of undeclared identifier 'pa'
   74 |                 n = *pa++ ^ *pb++;
      |                      ^~
hamming.c:74:16: error: use of undeclared identifier 'pb'
   74 |                 n = *pa++ ^ *pb++;
      |                              ^~
7 errors generated.
make: *** [<builtin>: hamming.o] Error 1