common/Journald: include msg/msg_fmt.h

common/Journald: include msg/msg_fmt.h

so we can use the formatter defined for `entity_name_t`. in fmtlib v9,
it is required to define a specialization for the formatted type even
the type has an override of operator<<(). now that we already have a
formatter for `entity_name_t`, let's just use it.

this change should address the FTBFS when building with fmtlib v9.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
(cherry picked from commit 6e223a6802152fc689ecdbf38fb4a410123844c7)