Inherit alpha_netbsd_nat_target from nbsd_nat_target

gdb/ChangeLog:

	* alpha-bsd-nat.c (alpha_netbsd_nat_target): Inherit from
	nbsd_nat_target instead of inf_ptrace_target.
	* alpha-bsd-nat.c: Include "nbsd-nat.h", as we are now using
	nbsd_nat_target.