binarystore: Fix for newer boost

The API for .data() on endian types changes from returning `char *` to
`unsigned char *`. This fixes the library to build against both APIs.

Change-Id: I432769e3a3992678becb6121de89d7c00f7c2d85
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/binarystore.cpp b/binarystore.cpp
index e036e0a..b7c88a3 100644
--- a/binarystore.cpp
+++ b/binarystore.cpp
@@ -245,7 +245,8 @@
     /* Store as little endian to be platform agnostic. Consistent with read. */
     auto blobData = blob_.SerializeAsString();
     boost::endian::little_uint64_t sizeLE = blobData.size();
-    std::string commitData(sizeLE.data(), sizeof(sizeLE));
+    std::string commitData(reinterpret_cast<const char*>(sizeLE.data()),
+                           sizeof(sizeLE));
     commitData += blobData;
 
     try