Fix ext2fs_swap{16,32,64} for external applications on big-endian machines

Fix ext2fs_swap{16,32,64} for external applications on big-endian machines

The public header files depend on the the autoconf defines
WORDS_BIGENDIAN and HAVE_SYS_TYPES_H, so we add them to ext2_types.h
so that external programs which try to use ext2fs_swap*() will work
correctly on big-endian systems.  Fortunately, few if any programs
need to use this libext2's byte-swap functions directly.

Addresses-Debian-Bug: #484879

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>