gdbserver: btrace enums

gdbserver: btrace enums

Fixes:

 ../../../src/gdb/gdbserver/linux-low.c: In function ‘int linux_low_read_btrace(btrace_target_info*, buffer*, int)’:
 ../../../src/gdb/gdbserver/linux-low.c:6827:48: error: invalid conversion from ‘int’ to ‘btrace_read_type’ [-fpermissive]
    err = linux_read_btrace (&btrace, tinfo, type);
						 ^
 In file included from ../../../src/gdb/gdbserver/linux-low.c:98:0:
 ../../../src/gdb/gdbserver/../nat/linux-btrace.h:116:26: error:   initializing argument 3 of ‘btrace_error linux_read_btrace(btrace_data*, btrace_target_info*, btrace_read_type)’ [-fpermissive]
  extern enum btrace_error linux_read_btrace (struct btrace_data *btrace,
			   ^

The cyclic dependency the comment talks about is no longer relevant:
  https://sourceware.org/ml/gdb-patches/2015-10/msg00643.html

gdb/gdbserver/ChangeLog:
2015-10-29  Pedro Alves  <palves@redhat.com>

	* linux-low.c (linux_low_read_btrace): Change type of 'type'
	parameter.
	* server.c (handle_qxfer_btrace): Change type of 'type'
	local.
	* target.h (struct target_ops) <read_btrace>: Change type of
	'type' parameter.  Update comment.