sys/log: add format attribute to modlog_printf prototype

Add format(printf) attribute to modlog_printf to enable compile-time
checking that format strings match arguments.

Note that this will cause incorrect modlog_printf usage which
previously compiled successfully, to no longer compile.

Author: ehaas

sys/log/modlog/include/modlog/modlog.h

@@ -190,7 +190,7 @@ int modlog_foreach(modlog_foreach_fn *fn, void *arg);
  * @param level                 The severity of the log entry to write.
  * @param msg                   The "printf" formatted string to write.
  */
-void modlog_printf(uint8_t module, uint8_t level, const char *msg, ...);
+void modlog_printf(uint8_t module, uint8_t level, const char *msg, ...) __attribute((format(printf, 3, 4)));
 
 /**
  * @brief Writes a specified number of bytes as a text entry to the specified log module.