Move quick_symbol_functions to a new header

Move quick_symbol_functions to a new header

This introduces a new header, quick-symbol.h, and moves
quick_symbol_functions and related typedefs into it.

gdb/ChangeLog
2021-03-20  Tom Tromey  <tom@tromey.com>

	* symfile.h (symbol_compare_ftype, symbol_filename_ftype)
	(expand_symtabs_file_matcher_ftype)
	(expand_symtabs_symbol_matcher_ftype)
	(expand_symtabs_exp_notify_ftype, struct quick_symbol_functions):
	Move to quick-symbol.h.
	* quick-symbol.h: New file.