x86 mptable: Include <x86/legacvar.h> for legacy_get_pcibus().

Fixes:		b076d8d5 mptable_hostb: Use legacy_get_pcibus() to fetch PCI bus number.
MFC after:	1 week

sys/x86/x86/mptable.c

@@ -50,6 +50,7 @@ __FBSDID("$FreeBSD$");
 #include <dev/pci/pcib_private.h>
 #endif
 #include <x86/apicreg.h>
+#include <x86/legacyvar.h>
 #include <x86/mptable.h>
 #include <machine/frame.h>
 #include <machine/intr_machdep.h>