Set NetBSD xml syscall file name to syscalls/netbsd.xml

Set NetBSD xml syscall file name to syscalls/netbsd.xml

The syscall literal names are not stable on NetBSD and can change
once a syscall is versioned. Thus these names are internal to the
system and in GDB mostly descriptive, not intended to be a stable
interface with fixed names across GDB and NetBSD versions to track
certain syscalls.

gdb/ChangeLog:

	* nbsd-tdep.c: Include "xml-syscall.h".
	(nbsd_init_abi): Call `set_xml_syscall_file_name'.