gptzfsboot: Move zfsboot.c source into its sole consumer

Suggested by:	imp
Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D51597

stand/i386/gptzfsboot/Makefile

@@ -1,7 +1,7 @@
 .include <bsd.init.mk>
 
 .PATH:		${BOOTSRC}/i386/boot2 ${BOOTSRC}/i386/gptboot \
-		${BOOTSRC}/i386/zfsboot ${BOOTSRC}/i386/common \
+		${BOOTSRC}/i386/common \
 		${BOOTSRC}/common
 
 FILES=		gptzfsboot