backports: add header file for function memcmp

backports: add header file for function memcmp

Function memcmp is implicit declaration on kernel 4.9.88 ARM platform.
backport-include/keys/asymmetric-type.h uses function memcmp but its
caller compat/verification/verify.c and all its related header files
do not include string.h. The header file is usually included from
arch/arm/include/asm, but in this configuration, it doesn't. We need
to be safe and insure string.h is there.

Signed-off-by: Winnie Chang <winnie.chang@cypress.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>