arm64: fix the handling of DDB symbols in early boot

The code which installs the kernel symbol table wasn't being compiled
because the file did not include opt_ddb.h

Reviewed by:	kevans emaste
MFC after:	1 week
Differential Revision: https://reviews.freebsd.org/D43490

sys/arm64/arm64/machdep_boot.c

@@ -27,6 +27,7 @@
  */
 
 #include "opt_platform.h"
+#include "opt_ddb.h"
 
 #include <sys/param.h>
 #include <sys/systm.h>
@@ -48,6 +49,10 @@
 #include <dev/fdt/fdt_common.h>
 #endif
 
+#ifdef DDB
+#include <ddb/ddb.h>
+#endif
+
 extern int *end;
 static char *loader_envp;