Use child_path to determine if an object file is under a sysroot.

This fixes the case where the sysroot happens to end in a trailing
'/'.  Note that the path returned from child_path always skips over
the directory separator at the start of the base path, so a separator
must always be explicitly added before the base path.

gdb/ChangeLog:

	* symfile.c (find_separate_debug_file): Use child_path to
	determine if an object file is under a sysroot.