libroot: convert pthread_spinlock to C++

Change-Id: Ic65b624d343c2cdb4bbb138c94c0ea2f01f27139
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4006
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

src/system/libroot/posix/pthread/Jamfile

@@ -25,7 +25,7 @@ for architectureObject in [ MultiArchSubDirSetup ] {
 			pthread_mutexattr.c
 			pthread_once.cpp
 			pthread_rwlock.cpp
-			pthread_spinlock.c
+			pthread_spinlock.cpp
 			;
 	}
 }