Stop MinikinSkia.h relying on SkRefCnt.h for <string> and <string_view>

Test: Ran locally
Change-Id: If70fdf419504b89e3a3501cf7f2133277064d78c

libs/hwui/hwui/MinikinSkia.h

@@ -20,6 +20,8 @@
 #include <SkRefCnt.h>
 #include <cutils/compiler.h>
 #include <minikin/MinikinFont.h>
+#include <string>
+#include <string_view>
 
 class SkFont;
 class SkTypeface;