crypto: asymmetric_keys - signature.c does not need <module.h>

This file does not contain any modular related function calls.  So get
rid of module.h since it drags in a lot of other headers and adds to
the preprocessing load.  It does export some symbols though, so we'll
need to ensure it has export.h present instead.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: linux-crypto@vger.kernel.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>