vmm_instruction_emul.c: fix bhyve build

The __diagused macro was used to cure a "set but not used" warning. This
broke the build for bhyve since __diagused is only defined in the
kernel. Define __diagused when not building the kernel.

Fixes:          5241577a22 ("vmm: fix set but not used warning")
Reported by:    Jenkins

sys/amd64/vmm/vmm_instruction_emul.c

@@ -58,6 +58,7 @@ __FBSDID("$FreeBSD$");
 #include <string.h>
 #include <strings.h>
 #include <vmmapi.h>
+#define	__diagused
 #define	KASSERT(exp,msg)	assert((exp))
 #define	panic(...)		errx(4, __VA_ARGS__)
 #endif	/* _KERNEL */