sync: fix C.pthread_mutex_t struct typedef is missing in darwin (fix: #15491) (#15845)

vlib/sync/sync_darwin.c.v

@@ -30,6 +30,18 @@ fn C.pthread_cond_wait(voidptr, voidptr) int
 fn C.pthread_cond_timedwait(voidptr, voidptr, voidptr) int
 fn C.pthread_cond_destroy(voidptr) int
 
+[typedef]
+struct C.pthread_mutex_t {}
+
+[typedef]
+struct C.pthread_rwlock_t {}
+
+[typedef]
+struct C.pthread_rwlockattr_t {}
+
+[typedef]
+struct C.sem_t {}
+
 // [init_with=new_mutex] // TODO: implement support for this struct attribute, and disallow Mutex{} from outside the sync.new_mutex() function.
 [heap]
 pub struct Mutex {

vlib/v/tests/str_gen_test.v

@@ -1,3 +1,5 @@
+import sync
+
 fn test_array_of_floats() {
 	// f64 array
 	aa := [1.2, 3.4, 5.67]
@@ -444,3 +446,20 @@ fn test_fixed_array_of_function() {
 	println(a)
 	assert '$a' == '[fn (string), fn (string)]'
 }
+
+struct CTypeDefStruct {
+	mutex &sync.Mutex
+}
+
+fn test_c_struct_typedef() {
+	$if macos || linux {
+		c := CTypeDefStruct{
+			mutex: sync.new_mutex()
+		}
+		assert c.str() == r'CTypeDefStruct{
+    mutex: &sync.Mutex{
+        mutex: pthread_mutex_t{}
+    }
+}'
+	}
+}