package/optee-client: fix build with gcc 4.8

Fix the following build failure with optee-client 3.11.0 and gcc 4.8:

/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 = { };
         ^

Build failure is raised since version 3.10.0 and
https://github.com/OP-TEE/optee_client/commit/c3279f424348bdd3378d236fd038fdd7bc76e10a
https://github.com/OP-TEE/optee_client/commit/e88c264ba358941d2166b18eeac448e617ffa887
https://github.com/OP-TEE/optee_client/commit/85a7ea7861339356b39c73926f2b5cb9added45b

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



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