Add missing libc++ includes am: 27b3545620

Original change: https://android-review.googlesource.com/c/platform/frameworks/minikin/+/2902171

Change-Id: Icb1f1279f12f28c209cb61c87b9851fbe664f91b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>
diff --git a/include/minikin/SystemFonts.h b/include/minikin/SystemFonts.h
index 4ef2705..518eb10 100644
--- a/include/minikin/SystemFonts.h
+++ b/include/minikin/SystemFonts.h
@@ -17,6 +17,7 @@
 #ifndef MINIKIN_SYSTEM_FONTS_H
 #define MINIKIN_SYSTEM_FONTS_H
 
+#include <functional>
 #include <map>
 #include <memory>
 #include <mutex>