Fixed GCC warning caused by MQTTClient_connectOptions_initializer constant missing a value for the 'returned' sub-structure of MQTTClient_connectOptions structure.

GCC check throwing the warning: 'missing-field-initializers', enabled by option -Wextra
GCC version 4.7.2

Error message:
warning: missing initializer for field 'returned' of 'MQTTClient_connectOptions' [-Wmissing-field-initializers]
MQTTClient_connectOptions client_opt = MQTTClient_connectOptions_initializer;
^
Signed-off-by: Sylvain Miermont <s.miermont@idosens.com>