Public header file of esp-modbus (freemodbus/port/port.h) includes
esp_timer.h, which is provided by esp_timer component. Currently
this works without declaring a dependency on esp_timer because
of a transitive dependency on esp_timer via freertos component.
This will be changed in IDF v5.0, where freertos will no longer
have a public dependency on esp_timer. This commit makes esp-modbus
compatible with that future change.