s3/registry: Fix func cast error (diff in params size_t vs uint32)

s3/registry: Fix func cast error (diff in params size_t vs uint32)

reg_parse_callback_key_t alread has size_t for number of elemens in key
but
reg_parse_callback_val_t uses uint32_t for length of value data.

Many internal function seem to take size_t as data len (see reg_format_value
etc.) It seems changing the signature of the callback aligns it more closely
with the existing api.

../../source3/registry/reg_format.c:438:10: error: cast between incompatible function types from ‘int (*)(reg_format *, const char *, uint32_t,  const uint8_t *, size_t)’ {aka ‘int (*)(struct reg_format *, const char *, unsigned int,  const unsigned char *, long unsigned int)’} to ‘int (*)(void *, const char *, uint32_t,  const uint8_t *, uint32_t)’ {aka ‘int (*)(void *, const char *, unsigned int,  const unsigned char *, unsigned int)’} [-Werror=cast-function-type]
   .val = (reg_parse_callback_val_t)&reg_format_value,

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>