Fix build from previous commit

I unfortunately got on an unnamed branch and pushed the wrong bits

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/mbuf.c b/mbuf.c
index d479cb7..799a6c8 100644
--- a/mbuf.c
+++ b/mbuf.c
@@ -64,7 +64,7 @@
     m->m_flags = (flags | M_USEDLIST);
 
     /* Initialise it */
-    m->m_size = SLIRP_MSIZE - offsetof(struct m_hdr, m_dat);
+    m->m_size = SLIRP_MSIZE - offsetof(struct mbuf, m_dat);
     m->m_data = m->m_dat;
     m->m_len = 0;
     m->m_nextpkt = NULL;