gdb: make symfile_segment_data::segment_info an std::vector

gdb: make symfile_segment_data::segment_info an std::vector

Change the symfile_segment_data::segment_info array to be an
std::vector.  No functional changes are expected.

gdb/ChangeLog:

	* symfile.h (struct symfile_segment_data)
	<~symfile_segment_data>: Remove.
	<segment_info>: Change to std::vector.
	* symfile.c (default_symfile_segments): Update.
	* elfread.c (elf_symfile_segments): Update.