scripts: set proper OpenSSL include dir also for sign-file

Fixes: 2cea4a7a

 ("scripts: use pkg-config to locate libcrypto")
Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Cc: stable@vger.kernel.org # 5.6.x
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>