Moved coder headers to the header files.
diff --git a/coders/json.h b/coders/json.h
index 5ad55ec..dbd7c0f 100644
--- a/coders/json.h
+++ b/coders/json.h
@@ -16,6 +16,8 @@
 
 #include "coders/coders-private.h"
 
+#define MagickJSONHeaders
+
 #if defined(__cplusplus) || defined(c_plusplus)
 extern "C" {
 #endif