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.

Bug: b/23084776
TEST=mma

Change-Id: I652a71fafed6705cfa3ab3c6c5a89082087ef055
3 files changed