libckteec: replace { } initializer by { 0 }

Replace { } initializer by { 0 } to fix the following build failure with
gcc 4.8:

libckteec/src/pkcs11_processing.c: In function 'ck_create_object':
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


Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>