filedef.h comment about file_timestamp_sprintf of by 10

[Rob Franken]

Dear Make maintainers,

While debugging some superfluous rebuilds in a project I wanted to
quickly hack in some printing of file_timestamps on out of date files.
When doing that I found the usefull helperfunction
file_timestamp_sprintf and since I knew that in the project I was
working on that before -999 or after 9999 wouldn't be a real problem I
just used the 19 that was mentioned in the comment. While that is
perhaps not the best idea to do I later found out that this 19 was of
by 10 since the nanoseconds were added. (Also my gut feeling to add 1
just in case the \0 end byte wasn't included was correct, but that might
just be a matter of personal preference, and not a bug.)

Kind Regards, 

=== snip: filedef.h:145 ===
/* Upper bound on length of string "YYYY-MM-DD HH:MM:SS.NNNNNNNNN"
   representing a file timestamp.  The upper bound is not necessarily 19,
   since the year might be less than -999 or greater than 9999.

=== snip ===

-- 
Rob