Include gdbtk.h to avoid declarations

Include gdbtk.h to avoid declarations

Once https://sourceware.org/ml/insight/2019-q4/msg00000.html lands,
we can just include gdbtk.h to get the declarations for
external_editor_command and gdbtk_test, instead of having to
declare them here in main.c.

gdb/ChangeLog:

2019-10-07  Christian Biesinger  <cbiesinger@google.com>

	* main.c (captured_main_1): Include gdbtk.h and remove declarations
	for external_editor_command and gdbtk_test.