treewide: fix invocation of c_avl_create

treewide: fix invocation of c_avl_create

Fixes the following warning on Solaris:

|c_avl_tree_t *c_avl_create (int (*compare) (const void *, const
void *));
|              ^  line 54, utils_avltree.h
|                 included in line 34, utils_vl_lookup.c
|
|  obj->by_type_tree = c_avl_create ((void *) strcmp);
|                                     ^  line 567,
utils_vl_lookup.c
E_ARG_INCOMPATIBLE_WITH_ARG_L, argument #1 is incompatible with
prototype:
        prototype: pointer to function(pointer to const void, pointer to
const void) returning int : "src/daemon/utils_avltree.h", line 54
        argument : pointer to void

I'll look into writing a generic function to compare avl keys so
we don't need to do all the casting.