Fix include statements for qemu-common.h

* qemu-common.h is not a system include file, so it should be included
  with "" instead of <>. Otherwise incremental builds might fail
  because only local include files are checked for changes.

* linux-user/syscall.c included the file twice.

Cc: Riku Voipio <riku.voipio@iki.fi>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
diff --git a/nbd.h b/nbd.h
index 96f77fe..273cfa1 100644
--- a/nbd.h
+++ b/nbd.h
@@ -21,7 +21,7 @@
 
 #include <sys/types.h>
 
-#include <qemu-common.h>
+#include "qemu-common.h"
 
 #include "block_int.h"