Include the x86-bsd-nat.h header and fix the NetBSD/i386 build

Include the x86-bsd-nat.h header and fix the NetBSD/i386 build

In file included from ../../gdb/i386-nbsd-nat.c:26:
../../gdb/i386-bsd-nat.h:34:53: error: expected template-name before '<' token
   34 | class i386_bsd_nat_target : public x86bsd_nat_target<BaseTarget>
      |                                                     ^
../../gdb/i386-bsd-nat.h:34:53: error: expected '{' before '<' token
../../gdb/i386-nbsd-nat.c:74:45: error: aggregate 'i386_bsd_nat_target<nbsd_nat_target> the_i386_nbsd_nat_target' has incomplete type and cannot be defined
   74 | static i386_bsd_nat_target<nbsd_nat_target> the_i386_nbsd_nat_target;
      |                                             ^~~~~~~~~~~~~~~~~~~~~~~~

gdb/ChangeLog:

	* i386-bsd-nat.c: Include "x86-bsd-nat.h".