Make the class name in the definition match the declaration

Fixes a compile error because the class is actually called
arm_netbsd_nat_target.

gdb/ChangeLog:

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

	* arm-nbsd-nat.c (arm_nbsd_nat_target::fetch_registers): Rename to...
	(arm_netbsd_nat_target::fetch_registers): ...this.
	(arm_nbsd_nat_target::store_registers): Rename to...
	(arm_netbsd_nat_target::store_registers): ...this.

Change-Id: Ibebfab9edeff48f54c32d0745afda1d74d31de92