Move the definition of M_MEMDESC to a non-optional file. This allows

kernels configurations without the 'mem' device to compile.

sys/amd64/amd64/amd64_mem.c

@@ -112,6 +112,11 @@ static int amd64_mtrrtomrt[] = {
 
 #define MTRRTOMRTLEN (sizeof(amd64_mtrrtomrt) / sizeof(amd64_mtrrtomrt[0]))
 
+/*
+ * Used in /dev/mem drivers and elsewhere
+ */
+MALLOC_DEFINE(M_MEMDESC, "memdesc", "memory range descriptors");
+
 static int
 amd64_mtrr2mrt(int val)
 {

sys/amd64/amd64/mem.c

@@ -67,11 +67,6 @@ __FBSDID("$FreeBSD$");
 
 #include <machine/memdev.h>
 
-/*
- * Used in /dev/mem drivers and elsewhere
- */
-MALLOC_DEFINE(M_MEMDESC, "memdesc", "memory range descriptors");
-
 /* ARGSUSED */
 int
 memrw(struct cdev *dev, struct uio *uio, int flags)