mxssb: Add -Wextra to gcc

This will allow GCC to lint the sources even more.
Signed-off-by: Marek Vasut <marex@denx.de>

Makefile

-CFLAGS=`pkg-config --cflags libcrypto` -Wall
+CFLAGS=`pkg-config --cflags libcrypto` -Wall -Wextra
 LDFLAGS=`pkg-config --libs libcrypto`
 
 mxssb:	mxssb.c