ifdef out the use of usesServerSideEncryption

That field exists in S3ResponseProperties on the master branch of the
libs3 repo, but not on the "2.0" branch that Ubuntu 18.04 packages, so
the minimal usage of that field has been #ifdef'd out.  If it's really
desired, define the HAS_USESSERVERSIDEENCRYPTION macro when compiling.

Fixes #2

src/s3util.c

@@ -281,9 +281,11 @@ S3Status responsePropertiesCallback (const S3ResponseProperties *properties, voi
  printf("x-amz-meta-%s: %s\n", properties->metaData[i].name,
  properties->metaData[i].value);
  }
+#ifdef HAS_USESSERVERSIDEENCRYPTION
  if (properties->usesServerSideEncryption) {
  printf("UsesServerSideEncryption: true\n");
  }
+#endif
 
  return S3StatusOK;
 }