[docs] Ignore the new HB_EXTERN decoration

Otherwise, almost all API functions are not extracted.

docs/Makefile.am

@@ -28,7 +28,8 @@ SCANGOBJ_OPTIONS=
 
 # Extra options to supply to gtkdoc-scan.
 # e.g. SCAN_OPTIONS=--deprecated-guards="GTK_DISABLE_DEPRECATED"
-SCAN_OPTIONS=--rebuild-types --deprecated-guards="HB_DISABLE_DEPRECATED"
+SCAN_OPTIONS=--rebuild-types --deprecated-guards="HB_DISABLE_DEPRECATED" \
+	--ignore-decorators="HB_EXTERN"
 
 # Header files or dirs to ignore when scanning. Use base file/dir names
 # e.g. IGNORE_HFILES=gtkdebug.h gtkintl.h private_code