constify search_symbols

constify search_symbols

This constifies the parameters to search_symbols and fixes up the
fallout.

Tested by rebuilding.

2014-06-30  Tom Tromey  <tromey@redhat.com>

	* symtab.c (operator_chars): Make parameters and return type
	const.
	(file_matches): Make "files" const.
	(struct search_symbols_data) <files>: Now const.
	(search_symbols): Make "regexp" and "files" parameters const.
	Update.
	(symtab_symbol_info): Remove cast.
	(rbreak_command): Update.
	* symtab.h (search_symbols): Update.