enable KGDB causes the build failure

[Rui-Xiang Guo <rxg%lavabit.com@localhost>]

Hi,
The build.sh runs on amd64 10.0 for cross compilation of evbarm GENERIC.
DDB, DDB_ONPANIC=1, DDB_HISTORY_SIZE=100 are commented out and KGDB is enabled.

--- db_interface.o ---
/home/rxg/10.0/usr/src/sys/arch/arm/arm32/db_interface.c:77:5: error: conflicting type qualifiers for 'db_active'
   77 | int db_active = 0;
      |     ^~~~~~~~~
In file included from /home/rxg/10.0/usr/src/sys/arch/arm/arm32/db_interface.c:59:
/home/rxg/10.0/usr/src/sys/ddb/db_active.h:41:18: note: previous definition of 'db_active' was here
   41 | static const int db_active __unused = 0;
      |                  ^~~~~~~~~

*** Failed target: db_interface.o

Do I miss something?

-rxg