2.8.1 fix global_timestamp: symbol not found on loading libplugin-mqtt.so

[Lyon1994]

In openwrt, after compiling neuron and running it, an error occurred when loading libplugin-mqtt.so stating global_timestamp: symbol not found. According to the results of objdump, it was discovered that global_timestamp is defined in main.c but cannot be found in libneuron-base.so. Therefore, global_timestamp needs to be moved to libneuron-base.so for definition.