log_manager: fix constexpr string length
log_manager: fix constexpr string length

'strlen' is not actually defined to be constexpr, but GCC has
an extension which allows it to be calculated at compile-time,
which clang does not support.  Switch to use string_view instead.

Signed-off-by: Patrick Williams <cbb7353e6d953ef360baf960c122346276c6e320@stwcx.xyz>
Change-Id: Ic18972a4d97d9ba17ac0530e3914b5f346381c7f

openbmc/phosphor-logging,openbmc/phosphor-logging,openbmc/phosphor-logging,openbmc/phosphor-logging