iodine: workaround incomplete musl nameser.h header

iodine: workaround incomplete musl nameser.h header

The arpa/nameser.h header of musl libc indirectly depends on the endian.h
header but fails to explicitely include it to properly define
`__BYTE_ORDER` and `__BIG_ENDIAN` prior to declaring the DNS `HEADER`
structure.

When both the appropriate `__BYTE_ORDER` and `__BIG_ENDIAN` defines are
unset, the `#if __BYTE_ORDER == __BIG_ENDIAN` condition in `nameser.h`
evaluates to true, causing it to declare a bad (big endian) DNS packet
header structure on little endian systems.

Work around this musl bug by forcibly passing `-include endian.h` through
the `osflags` file.

An upstream fix for musl libc has been submitted with
http://www.openwall.com/lists/musl/2017/12/04/3

This should solve iodine packet corruption on little endian musl systems
reported at
http://lists.infradead.org/pipermail/lede-dev/2017-November/010085.html

Signed-off-by: Jo-Philipp Wich <jo@mein.io>