Fix build with libtiff 4.4.0 by not using a private libtiff API.
Patch by Kurt Schwehr: https://sourceforge.net/p/freeimage/discussion/36109/thread/2018fdc6e7/

diff -ru a/Source/Metadata/XTIFF.cpp b/Source/Metadata/XTIFF.cpp
--- a/Source/Metadata/XTIFF.cpp
+++ b/Source/Metadata/XTIFF.cpp
@@ -749,7 +749,7 @@
 				continue;
 			}
 			// type of storage may differ (e.g. rationnal array vs float array type)
-			if((unsigned)_TIFFDataSize(tif_tag_type) != FreeImage_TagDataWidth(tag_type)) {
+			if((unsigned)TIFFFieldSetGetSize(fld) != FreeImage_TagDataWidth(tag_type)) {
 				// skip tag or _TIFFmemcpy will fail
 				continue;
 			}