Address type mismatch warnings on riscv64

[ellert]

The riscv64 dispatcher code uses the same PROVIDER_INFO macro as the aarch64 dispatcher and have the same kind of warnings during compilation:

igzip/riscv64/igzip_multibinary_riscv64_dispatcher.c:39:24: warning: type of 'adler32_base' does not match original declaration [-Wlto-type-mismatch]
   39 |                 return PROVIDER_BASIC(adler32);
      |                        ^
igzip/adler32_base.c:34:1: note: return value type mismatch
   34 | adler32_base(uint32_t adler32, uint8_t *start, uint64_t length)
      | ^
igzip/adler32_base.c:34:1: note: type 'uint32_t' should match type 'void'
igzip/adler32_base.c:34:1: note: 'adler32_base' was previously declared here

This commit introduces the same correction for riscv64.

See #313 for details.