libext2fs: export the library include path

libext2fs: export the library include path

When adding 'libext2fs' as a library dependency from any other binary,
the binary needs to find the library headers. Users of this library
should not hard-code the include path themselves. This patch exports the
library headers on all three version of the libext2fs library and
removes the redundant include path from executables in e2fsprogs.

Addresses-Google-Bug: #23084776
TEST=mma

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