MINOR: stktable: fix potential build issue in smp_to_stkey

smp_to_stkey() uses an ambiguous cast from 64bit integer to 32 bit
unsigned integer. While it is intended, let's make the cast less
ambiguous by explicitly casting the right part of the assignment to the
proper type.

This should fix GH haproxy#2838

src/stick_table.c

@@ -1500,7 +1500,7 @@ struct stktable_key *smp_to_stkey(struct sample *smp, struct stktable *t)
 		/* The stick table require a 32bit unsigned int, "sint" is a
 		 * signed 64 it, so we can convert it inplace.
 		 */
-		*_sint = smp->data.u.sint;
+		*_sint = (uint)smp->data.u.sint;
 		static_table_key.key = _sint;
 		static_table_key.key_len = 4;
 		break;