package/libkcapi: disable -Werror

Fix the following musl build failure (disabling -Werror seems to be the
only "solution": https://github.com/smuellerDD/libkcapi/issues/136):

lib/kcapi-kernel-if.c: In function '_kcapi_common_send_meta':
lib/kcapi-kernel-if.c:212:12: error: unsigned conversion from 'long int' to 'long unsigned int' changes value from '-4' to '4294967292' [-Werror=sign-conversion]
  212 |   header = CMSG_NXTHDR(&msg, header);
      |            ^~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/c789d43e71e6075e297bef58d888228a89055bbc



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