fix(modem): Support for custom modules with C-API

MAJOR CHANGE: Added support for implementing user defined modules in standard C-API

components/esp_modem/CMakeLists.txt

@@ -42,6 +42,9 @@ set_target_properties(${COMPONENT_LIB} PROPERTIES
     CXX_EXTENSIONS ON
 )
 
+if(CONFIG_ESP_MODEM_ADD_CUSTOM_MODULE)
+    idf_component_optional_requires(PUBLIC main)
+endif()
 
 if(${target} STREQUAL "linux")
     # This is needed for ESP_LOGx() macros, as integer formats differ on ESP32(..) and x64