8303915: javadoc build failure after JDK-8294959

Reviewed-by: jjg

Author: asotona

make/Docs.gmk

@@ -110,12 +110,14 @@ JAVA_WARNINGS_ARE_ERRORS ?= -Werror
 JAVADOC_OPTIONS := -use -keywords -notimestamp \
     -encoding ISO-8859-1 -docencoding UTF-8 -breakiterator \
     -splitIndex --system none -javafx --expand-requires transitive \
+    --enable-preview -source $(JDK_SOURCE_TARGET_VERSION) \
     --override-methods=summary
 
 # The reference options must stay stable to allow for comparisons across the
 # development cycle.
 REFERENCE_OPTIONS := -XDignore.symbol.file=true -use -keywords -notimestamp \
     -encoding ISO-8859-1 -breakiterator -splitIndex --system none \
+    --enable-preview -source $(JDK_SOURCE_TARGET_VERSION) \
     -html5 -javafx --expand-requires transitive
 
 # Should we add DRAFT stamps to the generated javadoc?