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

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 <[email protected]>
Signed-off-by: Randy Dunlap <[email protected]>
Cc: Mimi Zohar <[email protected]>
Cc: [email protected]
Link: http://kisskb.ellerman.id.au/kisskb/head/13396/
Signed-off-by: James Morris <[email protected]>