Fix our x86 mfence macro.

The newly-enabled debugging caught this deviation from the usual style.

Change-Id: I322e6f5d60102103e8970a4c851da656eb80d6e1
diff --git a/src/assembler_x86.h b/src/assembler_x86.h
index e87ce34..2fc486d 100644
--- a/src/assembler_x86.h
+++ b/src/assembler_x86.h
@@ -431,6 +431,8 @@
   X86Assembler* lock();
   void cmpxchgl(const Address& address, Register reg);
 
+  void mfence();
+
   X86Assembler* fs();
 
   //