package/uclibc: fix preadv/pwritev offset argument type

preadv/pwritev don't provide separate version for 64-bit wide off_t,
and default to 32-bit wide off_t, which results in a mismatch between
declaration and definition for user programs built with
-D_FILE_OFFSET_BITS=64.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>