Unbreak Linux build

  commit 71f4effc
  Author: Alexander Graf <agraf@suse.de>
  Date:   Fri Oct 30 22:27:00 2009 +0100

      Unbreak tap compilation on OS X

Broke the build on Linux due to a bad #if guard
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

net/tap-linux.h

@@ -17,7 +17,7 @@
 #define QEMU_TAP_H
 
 #include <stdint.h>
-#ifdef __LINUX__
+#ifdef __linux__
 
 #include <linux/ioctl.h>