util: hashtable: make hashing prototypes match

util: hashtable: make hashing prototypes match

It seems nobody's using the string hashing function. If you try to
pass it directly to the hashtable creation function, you'll get
compiler warning for non matching prototypes. Let's make them match.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>