clang: suppress warnings

Cast to "char *" explicitly.

    lib/hash.c:1321:16: warning: returning
          'uint8_t [4]' from a function with result type 'char *' converts between
          pointers to integer types with different sign [-Wpointer-sign]
            return entry->io_entry.key.buf;
                   ^~~~~~~~~~~~~~~~~~~~~~~
    lib/hash.c:1327:16: warning: returning
          'uint8_t [8]' from a function with result type 'char *' converts between
          pointers to integer types with different sign [-Wpointer-sign]
            return entry->tiny_entry.key.buf;
                   ^~~~~~~~~~~~~~~~~~~~~~~~~
    lib/hash.c:1334:14: warning: returning
          'uint8_t [4]' from a function with result type 'char *' converts between
          pointers to integer types with different sign [-Wpointer-sign]
          return entry->plain_entry.key;
                 ^~~~~~~~~~~~~~~~~~~~~~
    lib/hash.c:1336:14: warning: returning
          'uint8_t [1]' from a function with result type 'char *' converts between
          pointers to integer types with different sign [-Wpointer-sign]
          return entry->rich_entry.key_and_value;
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~