Change memset in fake_op to atap_memset.

We should use atap_memset that does not depend on string.h for
fake_atap_ops.

Change-Id: I111751d7b403f54d7c456a527bcee240c3f2af08
tests: Host unit tests compile and pass.
diff --git a/atap/test/fake_atap_ops.cpp b/atap/test/fake_atap_ops.cpp
index 2fa1f35..b5d106d 100644
--- a/atap/test/fake_atap_ops.cpp
+++ b/atap/test/fake_atap_ops.cpp
@@ -33,7 +33,7 @@
 
 AtapResult FakeAtapOps::read_product_id(
     uint8_t product_id[ATAP_PRODUCT_ID_LEN]) {
-  memset(product_id, 0x00, ATAP_PRODUCT_ID_LEN);
+  atap_memset(product_id, 0x00, ATAP_PRODUCT_ID_LEN);
   return ATAP_RESULT_OK;
 }