Drop superfluous enum forward declaration (GH-17455)

Besides that it is not needed, it is not proper C, and Clang warns that
"forward references to 'enum' types are a Microsoft extension"
(`-Wmicrosoft-enum-forward-reference`).

Author: cmb69

ext/mbstring/mbstring.c

@@ -4469,7 +4469,6 @@ PHP_FUNCTION(mb_send_mail)
 	bool suppress_content_transfer_encoding = false;
 
 	char *p;
-	enum mbfl_no_encoding;
 	const mbfl_encoding *tran_cs,	/* transfer text charset */
 						*head_enc,	/* header transfer encoding */
 						*body_enc;	/* body transfer encoding */