fix a compilation bug

This patch fixes this:

RayPlatform/store/KeyValueStore.cpp:76:31: error: 'memcpy' was not
declared in this scope

Reported-by: Fangfang Xia <fangfang@anl.gov>
Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>