atftp: fix compile with gcc5

package/atftp/Makefile

@@ -28,7 +28,7 @@ $(eval $(call PKG_template,ATFTP,atftp,${PKG_VERSION}-${PKG_RELEASE},${PKG_DEPEN
 $(eval $(call PKG_template,ATFTPD,atftpd,${PKG_VERSION}-${PKG_RELEASE},${PKG_DEPENDS},${PKGSD_ATFTPD},${PKG_SECTION}))
 
 CONFIGURE_ARGS+=	--disable-libpcre
-MAKE_FLAGS+=		CFLAGS="${TARGET_CFLAGS} -Wall -D_REENTRANT -fno-inline"
+MAKE_FLAGS+=		CFLAGS="${TARGET_CFLAGS} -Wall -D_REENTRANT -fno-inline -std=gnu89"
 
 atftp-install:
 	$(INSTALL_DIR) $(IDIR_ATFTP)/usr/bin