[modulemap] use textual header for character definitions

Both CSSCodePoints.h and HTMLTokenizerCharacters.h define the same
symbols.
They are never included by the same compilation unit, but when they are
compiled into the same module, they create a conflict.
Fix this conflict by using `textual header`.

Sources/include/module.modulemap

@@ -5,7 +5,7 @@ module HTMLKit {
 	export *
 
 	explicit module Private {
-		header "CSSCodePoints.h"
+		textual header "CSSCodePoints.h"
 		header "CSSInputStream.h"
 		header "HTMLCharacterToken.h"
 		header "HTMLCommentToken.h"
@@ -22,7 +22,7 @@ module HTMLKit {
 		header "HTMLTagToken.h"
 		header "HTMLToken.h"
 		header "HTMLTokenizer.h"
-		header "HTMLTokenizerCharacters.h"
+		textual header "HTMLTokenizerCharacters.h"
 		header "HTMLTokenizerEntities.h"
 		header "HTMLTokenizerStates.h"
 		header "HTMLTokens.h"