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_matcher.h b/include/android/font_matcher.h
index 63b0328..60ff95e 100644
--- a/include/android/font_matcher.h
+++ b/include/android/font_matcher.h
@@ -117,11 +117,12 @@
     AFAMILY_VARIANT_ELEGANT = 2,
 };
 
+struct AFontMatcher;
 /**
  * AFontMatcher performs match operation on given parameters and available font files.
  * This matcher is not a thread-safe object. Do not pass this matcher to other threads.
  */
-struct AFontMatcher;
+typedef struct AFontMatcher AFontMatcher;
 
 /**
  * Select the best font from given parameters.