arc: Add inclusion of "gdbarch.h" in "arc-tdep.h"

arc: Add inclusion of "gdbarch.h" in "arc-tdep.h"

The "arc-tdep.h" makes use of definitions in "gdbarch.h", but it
does not include it explicitly.  I have piggy backed this fix
in another commit [1], but I was asked to do it separately [2].

[1] arc: Add hardware loop detection
https://sourceware.org/pipermail/gdb-patches/2020-July/170800.html

[2] Simon's remarks to "arc: Add hardware loop detection"
https://sourceware.org/pipermail/gdb-patches/2020-August/170993.html

gdb/ChangeLog:

	* arc-tdep.h: Include "gdbarch.h".