Use C99 flexible array instead of 1-byte trailing array

Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NSamuel Thibault <samuel.thibault@ens-lyon.org>

slirp/mbuf.h

@@ -81,11 +81,9 @@ struct mbuf {
 	Slirp *slirp;
 	bool	resolution_requested;
 	uint64_t expiration_date;
+	char   *m_ext;
 	/* start of dynamic buffer area, must be last element */
-	union {
-		char	m_dat[1]; /* ANSI don't like 0 sized arrays */
-		char	*m_ext;
-	};
+	char    m_dat[];
 };
 
 #define ifq_prev m_prev