Moved coder headers to the header files.
diff --git a/coders/rgb.h b/coders/rgb.h
index 7bcc20b..7958425 100644
--- a/coders/rgb.h
+++ b/coders/rgb.h
@@ -16,6 +16,8 @@
 
 #include "coders/coders-private.h"
 
+#define MagickRGBHeaders
+
 #if defined(__cplusplus) || defined(c_plusplus)
 extern "C" {
 #endif