Add EASYEXIF_COPYRIGHT constant

Author: Blake-Madden

exif.h

@@ -40,6 +40,9 @@
 
 namespace easyexif {
 
+// Copyright information.
+constexpr char EASYEXIF_COPYRIGHT[] = "Copyright (c) 2010-2015 Mayank Lahiri\nModified work Copyright (c) 2018-2023 Andy Maloney";
+
 enum ParseError : int {
   None = 0,       // Parse was successful
   NoJPEG = 1982,  // No JPEG markers found in buffer, possibly invalid JPEG file