build: silence compiler warning on BSD

Building on 64-bit FreeBSD 8.2 complained about a cast between
a pointer and a smaller integer.  Going through an intermediate
cast shuts up the compiler.

* src/util/threads-pthread.c (virThreadSelfID): Silence a warning.

src/util/threads-pthread.c

@@ -216,7 +216,7 @@ int virThreadSelfID(void)
     tid = syscall(SYS_gettid);
     return (int)tid;
 #else
-    return (int)(void *)pthread_self();
+    return (int)(intptr_t)(void *)pthread_self();
 #endif
 }