package/libkcapi: fix uclibc build

Fix the following uclibc build failure raised since bump to version
1.4.0 in commit 06a9dc35 and
https://github.com/smuellerDD/libkcapi/commit/12f19b9a1dd308117f83e8cb33e28e3c040710a0:

lib/kcapi-kernel-if.c: In function '_kcapi_common_send_meta':
lib/kcapi-kernel-if.c:196:26: error: conversion to 'int' from 'size_t' {aka 'unsigned int'} may change the sign of the result [-Werror=sign-conversion]
  196 |         msg.msg_iovlen = kcapi_downcast_int(iovlen);
      |                          ^~~~~~~~~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/eccf4b84670b5ef0fdd68b46338edf5043c7cc0d



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>