export symbol used in tests (#85)

Author: bretambrose

include/aws/s3/private/s3_util.h

@@ -44,7 +44,10 @@ extern const struct aws_byte_cursor g_s3_service_name;
 extern const struct aws_byte_cursor g_range_header_name;
 extern const struct aws_byte_cursor g_content_range_header_name;
 extern const struct aws_byte_cursor g_accept_ranges_header_name;
+
+AWS_S3_API
 extern const struct aws_byte_cursor g_acl_header_name;
+
 extern const struct aws_byte_cursor g_post_method;
 extern const struct aws_byte_cursor g_delete_method;
 extern const uint32_t g_s3_max_num_upload_parts;