Define _KERNTYPES in arm-nbsd-nat.c

Fixes the below compile error on ARM NetBSD 9.0_RC1 (the only version I
tested).  types.h does not define register_t by default.

We already use this define elsewhere, notably in bsd-kvm.c.

In file included from ../../gdb/arm-nbsd-nat.c:28:
/usr/include/machine/frame.h:54:2: error: unknown type name 'register_t'; did you mean '__register_t'?
        register_t tf_spsr;
        ^
/usr/include/machine/types.h:77:14: note: '__register_t' declared here
typedef int             __register_t;
                        ^

There are other compile errors that this does not fix.

gdb/ChangeLog:

2020-01-24  Christian Biesinger  <cbiesinger@google.com>

	* arm-nbsd-nat.c: Define _KERNTYPES to get the declaration of
	register_t.

Change-Id: I82c21d38189ee59ea0af2538ba84b771d268722e