[BUILD] Guard '#pragma unroll' with COMPILING_FOR_MIN_SIZE
Summary: Disable unroll hints when COMPILING_FOR_MIN_SIZE is on. We were seeing hundreds of errors in the build because the optimization was not being performed.
Test Plan: Smoke builds
Differential Revision: D23513255
fbshipit-source-id: 87da2fdc3c1146e8ffcacf14a49d5151d313f367
