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/font.h b/include/android/font.h
index 45eb81a..0225725 100644
--- a/include/android/font.h
+++ b/include/android/font.h
@@ -87,10 +87,11 @@
     AFONT_WEIGHT_MAX = 1000
 };
 
+struct AFont;
 /**
  * AFont provides information of the single font configuration.
  */
-struct AFont;
+typedef struct AFont AFont;
 
 /**
  * Close an AFont.