arm64 kernel complile fail with nooption KDB

[Andreas Schwarz]

Building arm64 kernel fails, when KDB is undefined. This is caused by a commit 10 days ago,
kdb_trap() is used outside of a "#ifdef KDB" condition. 

Have a look at:

  https://svnweb.freebsd.org/base/head/sys/arm64/arm64/trap.c?r1=326227&r2=326312


--- trap.o ---
/usr/src/sys/arm64/arm64/trap.c:326:3: error: implicit declaration of function 'kdb_trap' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
                kdb_trap(exception, 0,
                ^
/usr/src/sys/arm64/arm64/trap.c:326:3: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
2 errors generated.
*** [trap.o] Error code 1

make[2]: stopped in /usr/obj/usr/src/arm64.aarch64/sys/PINE64-ASC
1 error

make[2]: stopped in /usr/obj/usr/src/arm64.aarch64/sys/PINE64-ASC
*** [buildkernel] Error code 2

make[1]: stopped in /usr/src
1 error

make[1]: stopped in /usr/src
*** [buildkernel] Error code 2

make: stopped in /usr/src
1 error

make: stopped in /usr/src


-asc