package/optee-client: disable -Werror

package/optee-client: disable -Werror

Disable -Werror thanks to CFG_WERROR which is available since version
3.3.0 and
https://github.com/OP-TEE/optee_client/commit/5355fdb841bce4f7cce3dd37fc31fa91bd625c98
to fix the following build failure with optee-client 3.11.0:

/home/giuliobenetti/autobuild/run/instance-2/output-1/build/optee-client-3.11.0/libckteec/src/pkcs11_processing.c: In function 'ck_create_object':
/home/giuliobenetti/autobuild/run/instance-2/output-1/build/optee-client-3.11.0/libckteec/src/pkcs11_processing.c:22:9: error: missing initializer for field 'buffer' of 'struct serializer' [-Werror=missing-field-initializers]
  struct serializer obj = { };
         ^

Fixes:
 - http://autobuild.buildroot.org/results/a3d663adb943aee814180f01d6e153b3309be962

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>