Makefile: Get rid of superfluous '+' character in date.

Commit bb3a974325f (Don't embed compile-time timestamps into generated
files) from a couple of weeks ago introduced a leading '+' in case
SOURCE_DATE_EPOCH is not set because DATE_FMT already contains the
'+' character for the argument to date(1), The superfluous '+'
becomes part of the generated man pages, which looks a bit weird.