scripts/kernel-doc: Fix build time warnings

As stated at Documentation/kbuild/llvm.rst, to make usage of ccache one
must set KBUILD_BUILD_TIMESTAMP=''. Setting this together with W=1
will trigger the following warning for every compiled file:

  date: invalid date ‘+%s’

This comes from kernel-doc script, that produces the following command
when KBUILD_BUILD_TIMESTAMP is empty:

  date -d"" +%s

That triggers the warning above. Add a space between the flag `-d` and
the string argument to fix date command and remove the warning.

Signed-off-by: André Almeida <andrealmeid@igalia.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241010225933.166436-1-andrealmeid@igalia.com