Change `label_symbols' to std::vector in linespec.c structures

Change `label_symbols' to std::vector in linespec.c structures

This patch converts linespec.c's linespec.label_symbols member from a
VEC to a std::vector.

gdb/ChangeLog:

	* linespec.c (struct linespec) <label_symbols>: Change type to
	std::vector.  Update all users.
	(find_label_symbols_in_block): Change `result' parameter to
	std::vector.  Update all callers.
	(find_label_symbols): Return std::vector.  Update all callers.