Move headers to subdir of include/, install missing headers

[ferdnyc]

The install process currently installs only squashfuse.h and squashfs_fs.h, directly into PREFIX/include/.

Unfortunately, because the contents of squashfuse.h is only:

squashfuse/squashfuse.h

Lines 28 to 33 in 0b48352

	#include "dir.h"
	#include "file.h"
	#include "fs.h"
	#include "traverse.h"
	#include "util.h"
	#include "xattr.h"

it is useless without those other files, and the installation is broken (cannot be compiled against).

This PR adds all of the listed headers, plus any #included by those headers, to the pkgconfigdir_HEADERS list, and sets pkgconfigdir = @includedir@/squashfuse. This means that all of the necessary headers will be installed in PREFIX/include/squashfuse/ and they can be referenced at that path to build software that links against the library.

The squashfuse.pc.in file is also updated to point includedir to the new subdirectory. (There may be a more clever way of writing the .pc.in file that makes use of @pkgconfigdir@ directly, but I couldn't find a way to make that work. It just kept passing through the template processing unreplaced.)