benchmarks/unixbench: Fix path to LICENSE_FILE

Accidently forgot to adjust path to LICENSE_FILE correctly causing
build to fail

Reported by:	meta
Approved by:	portmgr (blanket, build fix)

benchmarks/unixbench/Makefile

@@ -10,7 +10,7 @@ COMMENT=	BYTE magazine's Public Domain benchmark for UNIX
 WWW=		https://github.com/kdlucas/byte-unixbench
 
 LICENSE=	GPLv2
-LICENSE_FILE=	${WRKSRC}/LICENSE.txt
+LICENSE_FILE=	${WRKSRC}/../LICENSE.txt
 
 RUN_DEPENDS=	x11perf:x11/x11perf