libarchive: Do not use pthread.h API without header

Utilities/cmlibarchive/libarchive/archive_random.c

@@ -142,11 +142,10 @@ struct arc4_stream {
 	uint8_t s[256];
 };
 
-static pthread_mutex_t	arc4random_mtx = PTHREAD_MUTEX_INITIALIZER;
-
 #define	RANDOMDEV	"/dev/urandom"
 #define	KEYSIZE		128
 #ifdef HAVE_PTHREAD_H
+static pthread_mutex_t	arc4random_mtx = PTHREAD_MUTEX_INITIALIZER;
 #define	_ARC4_LOCK()	pthread_mutex_lock(&arc4random_mtx);
 #define	_ARC4_UNLOCK()  pthread_mutex_unlock(&arc4random_mtx);
 #else