Define _KERNTYPES in alpha-bsd-nat.c

Fixes build on NetBSD. types.h does not define register_t by default.

gdb/ChangeLog:

	* alpha-bsd-nat.c: Define _KERNTYPES to get the declaration of
	register_t.

gdb/alpha-bsd-nat.c

@@ -17,6 +17,8 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
+/* We define this to get types like register_t.  */
+#define _KERNTYPES
 #include "defs.h"
 #include "inferior.h"
 #include "regcache.h"