Typedef font structs

Use the `struct S; typedef struct S S;` pattern in NDK font headers.
This allows these headers to compile in C and matches the pattern used
by other NDK headers.

Change-Id: I912b2a517931eabbf2405f322ee1f5ddb53dd19f
diff --git a/include/android/system_fonts.h b/include/android/system_fonts.h
index b0bbb95..94484ea 100644
--- a/include/android/system_fonts.h
+++ b/include/android/system_fonts.h
@@ -87,13 +87,14 @@
 
 __BEGIN_DECLS
 
+struct ASystemFontIterator;
 /**
  * ASystemFontIterator provides access to the system font configuration.
  *
  * ASystemFontIterator is an iterator for all available system font settings.
  * This iterator is not a thread-safe object. Do not pass this iterator to other threads.
  */
-struct ASystemFontIterator;
+typedef struct ASystemFontIterator ASystemFontIterator;
 
 /**
  * Create a system font iterator.