risu.c: add missing <string.h> include for memset
risu.c: add missing <string.h> include for memset

Signed-off-by: Claudio Fontana <5fe97b97070bc75ff3c7a6b130e2dcc62cbb9b58@linaro.org>

hw-claudio/risu-aarch64,hw-claudio/risu-aarch64