uboot-ar71xx: fix musl host build

uboot-ar71xx: fix musl host build

On musl based distributions, u-boot 2010.03 fails to build with:

    u-boot-2010.03/include/u-boot/crc.h:29:50: error: unknown type name 'uint'
      uint32_t crc32 (uint32_t, const unsigned char *, uint);

The issue was fixed in the newer u-boot-2018.03 version, this commit
backports the change to the older version used by ar71xx/ath79.

Signed-off-by: Andy Walsh <andy.walsh44+github@gmail.com>
[add commit message from PR description]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>