mtd: add missing <endian.h> include

mtd: add missing <endian.h> include

trx.c in mtd would not include endian.h, so on systems that do not have
this header implicitly included from the other headers (like musl), both
__BYTE_ORDER and __BIG_ENDIAN would be undefined and thus 0, leading to
it always presuming a big-endian system. this would lead to issues when
running mtd fixtrx on little-endian systems, as it would never recognize
the TRX magic as result of the broken STORE32_LE() macro.

Signed-off-by: Shiz <hi@shiz.me>
Tested-by: Shiz <hi@shiz.me>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45896 3c298f89-4303-0410-b956-a3cf2f4a3e73