sign-file: fix build with CMS support disabled

Some versions of openssl might have the CMS feature disabled
LibreSSL disables this feature too
If the feature is disabled, fallback to PKCS7

In file included from scripts/sign-file.c:46:0:
/usr/x86_64-pc-linux-gnu/include/openssl/cms.h:62:2: error: #error CMS is disabled.
 #error CMS is disabled.

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
Signed-off-by: David Howells <dhowells@redhat.com>