docs/process/howto: Replace C89 with C11

docs/process/howto: Replace C89 with C11

Commit 8b522ed12638 ("Kbuild: move to -std=gnu11") updated
process/programming-language.rst, but failed to update
process/howto.rst.

Update howto.rst and resolve the inconsistency.

Fixes: 8b522ed12638 ("Kbuild: move to -std=gnu11")
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Federico Vaga <federico.vaga@vaga.pv.it>
Cc: Alex Shi <alexs@kernel.org>
Cc: Hu Haowen <src.res@email.cn>
Cc: Tsugikazu Shibata <shibata@linuxfoundation.org>
Link: https://lore.kernel.org/r/20221015092201.32099-1-akiyks@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>