Include missing header to get missing declarations

  CXX    amd64-bsd-nat.o
amd64-bsd-nat.c:42:1: error: no previous declaration void amd64bsd_fetch_inferior_registers(regcache*,  [-Werror=missing-declarations]
 amd64bsd_fetch_inferior_registers (struct regcache *regcache, int regnum)
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
amd64-bsd-nat.c:118:1: error: no previous declaration void amd64bsd_store_inferior_registers(regcache*,  [-Werror=missing-declarations]
 amd64bsd_store_inferior_registers (struct regcache *regcache, int regnum)
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Detected on NetBSD/amd64 9.99.49.

gdb/ChangeLog:

	* amd64-bsd-nat.c: Include amd64-bsd-nat.h".