Pass strcmp() to c_avl_create() as function pointer, not data one

Pass strcmp() to c_avl_create() as function pointer, not data one

Matches how strcmp() is passed to c_avl_create() elsewhere.

Reported by -Wpendantic:
------------------------------------
write_prometheus.c: In function ‘prom_init’:
write_prometheus.c:900:28: error: ISO C forbids conversion of function pointer to object pointer type [-Werror=pedantic]
  900 |     metrics = c_avl_create((void *)strcmp);
      |                            ^
write_prometheus.c:900:28: error: ISO C forbids passing argument 1 of ‘c_avl_create’ between function pointer and ‘void *’ [-Werror=pedantic]
  900 |     metrics = c_avl_create((void *)strcmp);
      |                            ^~~~~~~~~~~~~~
In file included from write_prometheus.c:30:
utils/avltree/avltree.h:54:34: note: expected ‘int (*)(const void *, const void *)’ but argument is of type ‘void *’
   54 | c_avl_tree_t *c_avl_create(int (*compare)(const void *, const void *));
      |
                                   ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
------------------------------------

Signed-off-by: Eero Tamminen <eero.t.tamminen@intel.com>