lib: Promote byte_conversions.h #include to hivex-internal.h

This patch addresses a build failure in OS X.  Running git-bisect on a
straightforward build (bootstrap, autogen.sh, configure, make, make
install) showed this as the "Bad commit:"
3e7c039799cddc45517350cc917eb10715f33fec

The issue is that hivex-internal.h uses le32toh in a static inline
function.  In case `configure` doesn't find le32toh, byte_conversions.h
defines it.  But hivex-internal.h doesn't include the safety definition.

OS X demonstrates this a problem.  Neither endian.h nor byteswap.h are
found with `configure` in OS X 10.8.5 (XCode 5), but the headers are
both found in Fedora 19 and Ubuntu 13.04.  This patch to configure.ac
further logs that only ntohl is available for byte swaps:

    @@ -153,6 +153,8 @@ AC_REPLACE_FUNCS([mmap])
     dnl Functions.
     AC_CHECK_FUNCS([bindtextdomain])

    +AC_CHECK_FUNCS([le32toh ntohl bswap_32 __bswap_32])
    +

(As an aside, it's curious that a missing byteswap.h didn't cause
hivex-internal.h to fail to build.)
(As another aside, this is an interesting example of lazy symbol
binding failing with an inline function.)

The problem is resolved by having hivex-internal.h include
byte_conversions.h.  This obviates most of the other direct inclusions
of byte_conversions.h.  (One persists under sh/.)

This patch builds and runs hivexml on images/large fine in Ubuntu 13.04
and Fedora 19.  It also allows builds to succeed in OS X, but doesn't
run hivexml for an unrelated reason.  The _iconv_open
symbol-not-found issue, that I thought was previously resolved
(491ba0f7a761c7ffd50e0eaa4d892f78d538eb2b), resurfaced.

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>