integrity/security: fix digsig.c build error with header file

security/integrity/digsig.c has build errors on some $ARCH due to a
missing header file, so add it.

  security/integrity/digsig.c:146:2: error: implicit declaration of function 'vfree' [-Werror=implicit-function-declaration]

Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: linux-integrity@vger.kernel.org
Link: http://kisskb.ellerman.id.au/kisskb/head/13396/


Signed-off-by: James Morris <james.morris@microsoft.com>