keys: remove needless modular infrastructure from ecryptfs_format

Even though the support can be modular, only one file needs to use
all the macros like MODULE_AUTHOR, MODULE_LICENSE etc.  Only the one
responsible for registering/removal with module_init/module_exit
needs to declare these.  In this case, that file is "encrypted.c"
and it already has the MODULE_LICENSE that we are removing here.

Since the file does EXPORT_SYMBOL, we add export.h - and build tests
show that module.h (which includes everything) was hiding an implicit
use of string.h - so that is added as well.

Cc: Mimi Zohar <zohar@linux.ibm.com>
Cc: David Howells <dhowells@redhat.com>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: linux-integrity@vger.kernel.org
Cc: keyrings@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: James Morris <james.morris@microsoft.com>