MODSIGN: linux/string.h should be #included to get memcpy()

linux/string.h should be #included in module_signing.c to get memcpy(),
lest the following occur:

    kernel/module_signing.c: In function 'mod_verify_sig':
    kernel/module_signing.c:57:2: error: implicit declaration of function 'memcpy' [-Werror=implicit-function-declaration]
      memcpy(&ms, mod + (modlen - sizeof(ms)), sizeof(ms));
      ^

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>