sfxge(4): fix build because of no declaration

Functions declared in mcdi_mon.h are implemented in mcdi_mon.c.
The build fails if compiler options require declaration before definition.

Sponsored by:   Solarflare Communications, Inc.
MFC after:      1 week
Differential Revision:  https://reviews.freebsd.org/D18246

sys/dev/sfxge/common/mcdi_mon.c

@@ -33,6 +33,7 @@ __FBSDID("$FreeBSD$");
 
 #include "efx.h"
 #include "efx_impl.h"
+#include "mcdi_mon.h"
 
 #if EFSYS_OPT_MON_MCDI