Improve consistency of '#error' directive messages

Remove the use of contractions and use proper punctuation in #error
directive messages that discourage the direct inclusion of header files.

Link: https://lkml.kernel.org/r/20241105032231.28833-1-natanielfarzan@gmail.com


Signed-off-by: Nataniel Farzan <natanielfarzan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>