refactor(gw_get_script_hash_by_prefix_fn): use const for prefix arg

[Flouse]

• fix error: invalid conversion

from 'int (*)(gw_context_t*, const uint8_t*, uint64_t, uint8_t*)' {aka 'int (*)(gw_context_t*, const unsigned char*, long unsigned int, unsigned char*)'} to 'gw_get_script_hash_by_prefix_fn' {aka 'int (*)(gw_context_t*, unsigned char*, long unsigned int, unsigned char*)'} [-fpermissive]
   ctx->sys_get_script_hash_by_prefix = sys_get_script_hash_by_prefix;
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

c/tests/../generator/../polyjuice_utils.h:156:51: error: invalid conversion from 'const uint8_t*' {aka 'const unsigned char*'} to 'uint8_t*' {aka 'unsigned char*'} [-fpermissive]
     ret = ctx->sys_get_script_hash_by_prefix(ctx, eth_address,