Keymaster: Fix unused variables

For build-system CFLAGS clean-up, fix unused variables.

Change-Id: I0004b7f05059fb9f9866aa23e54128f67c61ed60
diff --git a/key_blob.cpp b/key_blob.cpp
index 6b24355..08cf0d3 100644
--- a/key_blob.cpp
+++ b/key_blob.cpp
@@ -42,7 +42,7 @@
 const uint8_t BLOB_VERSION = 0;
 
 uint8_t* KeyBlob::Serialize(uint8_t* buf, const uint8_t* end) const {
-    const uint8_t* start = buf;
+    const uint8_t* start __unused = buf;
     *buf++ = BLOB_VERSION;
     buf = append_size_and_data_to_buf(buf, end, nonce(), NONCE_LENGTH);
     buf = append_size_and_data_to_buf(buf, end, encrypted_key_material(), key_material_length());