gdb/netbsd: add missing header file

I ran into this error when building GDB on NetBSD:

  CXX    netbsd-nat.o
netbsd-nat.c: In member function 'virtual bool nbsd_nat_target::info_proc(const char*, info_proc_what)':
netbsd-nat.c:314:3: error: 'gdb_argv' was not declared in this scope
   gdb_argv built_argv (args);
   ^~~~~~~~
netbsd-nat.c:314:3: note: suggested alternative: 'gdbarch'
   gdb_argv built_argv (args);
   ^~~~~~~~
   gdbarch
netbsd-nat.c:315:7: error: 'built_argv' was not declared in this scope
   if (built_argv.count () == 0)
       ^~~~~~~~~~
netbsd-nat.c:315:7: note: suggested alternative: 'buildargv'
   if (built_argv.count () == 0)
       ^~~~~~~~~~
       buildargv
gmake[2]: *** [Makefile:1893: netbsd-nat.o] Error 1

Fix this by adding the missing header file, as it is obvious.

Tested by rebuilding on NetBSD/amd64.