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 <[email protected]>
Acked-by: Randy Dunlap <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: andrealmeid

scripts/kernel-doc

@@ -160,7 +160,7 @@ my @export_file_list;
 
 my @build_time;
 if (defined($ENV{'KBUILD_BUILD_TIMESTAMP'}) &&
-    (my $seconds = `date -d"${ENV{'KBUILD_BUILD_TIMESTAMP'}}" +%s`) ne '') {
+    (my $seconds = `date -d "${ENV{'KBUILD_BUILD_TIMESTAMP'}}" +%s`) ne '') {
     @build_time = gmtime($seconds);
 } else {
     @build_time = localtime;