Fix exiv2.hpp if built with PNG support disabled (--without-zlib)

Fixed upstream in >=0.26

See also: http://dev.exiv2.org/issues/1103
Gentoo bug 1: https://bugs.gentoo.org/show_bug.cgi?id=552046
Gentoo bug 2: https://bugs.gentoo.org/show_bug.cgi?id=535836

Index: exiv2.hpp
===================================================================
--- a/include/exiv2/exiv2.hpp	(revision 3887)
+++ b/include/exiv2/exiv2.hpp	(revision 3888)
@@ -52,7 +52,9 @@
 #include "mrwimage.hpp"
 #include "orfimage.hpp"
 #include "pgfimage.hpp"
+#ifdef   EXV_HAVE_LIBZ
 #include "pngimage.hpp"
+#endif
 #include "preview.hpp"
 #include "properties.hpp"
 #include "psdimage.hpp"