cicuitpy_mpconfg.h: Define RUN_BACKGROUND_TASKS

In #2013, @danh says:
    My choice of where to put the semicolon is deliberate,
    so that we can say

        RUN_BACKGROUND_TASKS;

    not have a redundant semicolon, and not confuse C code formatting.

py/circuitpy_mpconfig.h

@@ -653,6 +653,7 @@ extern const struct _mp_obj_module_t ustack_module;
     NETWORK_ROOT_POINTERS \
 
 void run_background_tasks(void);
+#define RUN_BACKGROUND_TASKS (run_background_tasks())
 
 // TODO: Used in wiznet5k driver, but may not be needed in the long run.
 #define MICROPY_THREAD_YIELD()