Fix typo in macro definition

The macro atomic_exchange_explicit had a formal parameter declared as
"desired", but the definition  __c11_atomic_exchange_explicit(obj,
expected, order) ended up using a non declared parameter, "expected".

This would happen if the target system defines  __CLANG_ATOMICS during
processing of the header file.

Fixed the macro definition by substituting "expected" for "desired" to
match the name declared in the macro's parameters list.

lib/isc/unix/include/isc/stdatomic.h

@@ -142,7 +142,7 @@ typedef uintmax_t	   atomic_uintmax_t;
 	__c11_atomic_compare_exchange_weak_explicit(obj, expected, desired, \
 						    succ, fail)
 #define atomic_exchange_explicit(obj, desired, order) \
-	__c11_atomic_exchange_explicit(obj, expected, order)
+	__c11_atomic_exchange_explicit(obj, desired, order)
 #elif defined(__GNUC_ATOMICS) /* __atomic builtins */
 #define atomic_init(obj, desired)	 (*obj = desired)
 #define atomic_load_explicit(obj, order) __atomic_load_n(obj, order)