uthash.h: fix compiler warning -Wcast-qual

The macro HASH_ADD_KEYPTR_BYHASHVALUE casts keyptr to (char *) hence
violating const keys. As the key is used read-only, the UT_hash_handle
structure can be changed to const void *key, allowing the cast to
(const void *) instead.

This fixes compiler warnings emitted when -Wcast-qual is specified.