core/log: add @file doxygen command

Without this command the `log.h` documentation gets mangled with the
`core_util` documentation.

core/include/log.h

@@ -10,6 +10,7 @@
  * @addtogroup  core_util
  * @{
  *
+ * @file
  * @brief       System logging header
  *
  * This header offers a bunch of "LOG_*" functions that, with the default