Acknowledge the contributors of bug fixes applied to the EXIF support

Considering that the EXIF support is part of the core libpng library
(which excludes scripts, tests, and other independent programs), its
contributors should be acknowledged in the AUTHORS file.
diff --git a/AUTHORS b/AUTHORS
index 7b40082..e6a58d2 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -20,7 +20,9 @@
  * Mans Rullgard
  * Matt Sarett
  * Mike Klein
+ * Pascal Massimino
  * Paul Schmidt
+ * Qiang Zhou
  * Sam Bushell
  * Samuel Williams
  * Simon-Pierre Cadieux