Fix build breakage from previous commit

Fix build breakage from previous commit

In the previous commit, I forgot to adjust the prototypes of the
functions inside gdb/xml-syscall.c for the case when GDB is compiled
without XML support.

gdb/
2014-11-20  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR breakpoints/10737
	* xml-syscall.c (set_xml_syscall_file_name): Remove "const"
	modifier from "struct gdbarch" when compiling without Expat (XML)
	support.
	(get_syscall_by_number): Likewise.
	(get_syscall_by_name): Likewise.
	(get_syscall_names): Likewise.