scripts: Fix the remaining instance of ZLIB_INCLUDE_DIR
diff --git a/scripts/genout.cmake.in b/scripts/genout.cmake.in
index 01f12de..91d3bd4 100644
--- a/scripts/genout.cmake.in
+++ b/scripts/genout.cmake.in
@@ -20,7 +20,7 @@
 set(PNGLIB_MAJOR "@PNGLIB_MAJOR@")
 set(PNGLIB_MINOR "@PNGLIB_MINOR@")
 set(PNGLIB_VERSION "@PNGLIB_VERSION@")
-set(ZLIBINCDIR "@ZLIB_INCLUDE_DIR@")
+set(ZLIBINCDIR "@ZLIB_INCLUDE_DIRS@")
 
 set(PLATFORM_C_FLAGS)
 if(APPLE)