guest/gralloc: Fix build error

The error I was seeing with local build was

---
  device/google/cuttlefish_common/guest/hals/gralloc//legacy/gralloc_vsoc_priv.h:163:3: error: use of undeclared identifier 'memset'
     memset(out, 0, sizeof(*out));
     ^
  1 error generated.
---

Bug: None
Test: aosp_cf_x86_64-userdebug builds

Change-Id: I722da1c0913d910df7eb7e814199a06de80d47a8
Signed-off-by: Sandeep Patil <sspatil@google.com>