Fix symlinks paths handling in abipkgdiff

Fix symlinks paths handling in abipkgdiff

When elements of an RPMs are referred to using a path that contains
symlinks, we need to resolve the symlinks in order to be able to
compare those paths.  We are not doing this fully and so we are
hitting corner cases where things break down in subtle ways.

This patch fixes that.

	* include/abg-tools-utils.h (real_path): Declare new function.
	* src/abg-tools-utils.cc (real_path): Define it.
	* tools/abipkgdiff.cc (package::convert_path_to_relative): Use the
	new real_path function to consider real path (where symlinks are
	resolved) of the extraction directory of the package.
	(get_interesting_files_under_dir): Similarly, use the new
	real_path function to consider the real path of the directory we
	are exploring.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>