[gdb] Fix gdb build on mips64-linux

[gdb] Fix gdb build on mips64-linux

When compiling for mips64-linux, we get:
...
src/gdb/linux-nat-trad.c:139:12: error: ‘gdbarch_num_regs’ was not declared \
  in this scope
  139 |   regnum < gdbarch_num_regs (regcache->arch ());
...

Fix this by including gdbarch.h in linux-nat-trad.c, similar to commit
b1c896b365 "Fix gdb build on macOS".

Build on mips64-linux.

gdb/ChangeLog:

2019-08-15  Tom de Vries  <tdevries@suse.de>

	* linux-nat-trad.c: Include gdbarch.h.