gc-libgcrypt: fix undefined enum type in switch statement
Resolve the following build failure:
lib/gc-libgcrypt.c: In function 'gc_hash_open':
lib/gc-libgcrypt.c:317:5: error: case value '0' not in enumerated type
'Gc_hash_mode {aka enum Gc_hash_mode}' [-Werror=switch]
case 0:
^~~~
* lib/gc.h (enum Gc_hash_mode): Add value GC_NULL.
* lib/gc-libgcrypt.c (gc_hash_open): Use this enum value instead of 0.