Create xml-builtin.h to declare xml_builtins

Create xml-builtin.h to declare xml_builtins

xml-builtin.c only has character arrays and no dependencies, so this
creates a simple header file for that purpose so that gdbserver
can include that instead of re-declaring xml_builtin.

Despite the name, feature_to_c.sh is already specific to xml_builtins
(it hardcodes the variable name), so making it always output the
include for xml-builtin.h seems fine.

gdb/ChangeLog:

2019-10-16  Christian Biesinger  <cbiesinger@google.com>

	* Makefile.in: Add xml-builtin.h.
	* features/feature_to_c.sh: Add an include for xml-builtin.h
	to ensure that the compiler checks that the types match.
	* xml-builtin.h: New file.
	* xml-support.c (fetch_xml_builtin): Add missing const.
	* xml-support.h: Remove declaration of xml_builtins.

gdb/gdbserver/ChangeLog:

2019-10-16  Christian Biesinger  <cbiesinger@google.com>

	* server.c: Include xml-builtin.h.
	(get_xml_features): Don't declare xml_builtins here.

Change-Id: I806ef0851c43ead90b545a11794e41f5e5178436