logging: log_core: add extra space in missed strdup err msg

It improves readability in an error message:

before: <err> log: argument 0 in source storage log message "val %s"
missinglog_strdup().

after: <err> log: argument 0 in source storage log message "val %s"
missing log_strdup().

Signed-off-by: Bartosz Bilas <b.bilas@grinn-global.com>