Include SkParagraph headers only when the enable-skshaper flag is on (#9758)

These headers are currently causing errors in MSVC-based Windows builds.

third_party/txt/src/txt/font_collection.h

@@ -27,10 +27,13 @@
 #include "third_party/googletest/googletest/include/gtest/gtest_prod.h"  // nogncheck
 #include "third_party/skia/include/core/SkFontMgr.h"
 #include "third_party/skia/include/core/SkRefCnt.h"
-#include "third_party/skia/modules/skparagraph/include/FontCollection.h"
 #include "txt/asset_font_manager.h"
 #include "txt/text_style.h"
 
+#if FLUTTER_ENABLE_SKSHAPER
+#include "third_party/skia/modules/skparagraph/include/FontCollection.h"
+#endif
+
 namespace txt {
 
 class FontCollection : public std::enable_shared_from_this<FontCollection> {

third_party/txt/src/txt/paragraph_builder.cc

@@ -15,11 +15,14 @@
  */
 
 #include "paragraph_builder.h"
-#include "flutter/third_party/txt/src/skia/paragraph_builder_skia.h"
 #include "paragraph_builder_txt.h"
 #include "paragraph_style.h"
 #include "third_party/icu/source/common/unicode/unistr.h"
 
+#if FLUTTER_ENABLE_SKSHAPER
+#include "flutter/third_party/txt/src/skia/paragraph_builder_skia.h"
+#endif
+
 namespace txt {
 
 std::unique_ptr<ParagraphBuilder> ParagraphBuilder::CreateTxtBuilder(