devel/xxhash: Update to 0.8.1

While at it, switch to DISTVERSION as per recommendation in
Porters Handbook

Changelog: https://github.com/Cyan4973/xxHash/releases/tag/v0.8.1

PR:		260273
Approved by:	sunpoet (maintainer timeout, 4+ weeks)

devel/xxhash/Makefile

@@ -1,6 +1,6 @@
 PORTNAME=	xxhash
-PORTVERSION=	0.8.0
 DISTVERSIONPREFIX=	v
+DISTVERSION=	0.8.1
 CATEGORIES=	devel
 
 MAINTAINER=	sunpoet@FreeBSD.org
@@ -14,7 +14,7 @@ USES=		gmake
 USE_LDCONFIG=	yes
 TEST_TARGET=	check
 
-PLIST_SUB=	PORTVERSION=${PORTVERSION}
+PLIST_SUB=	DISTVERSION=${DISTVERSION}
 
 USE_GITHUB=	yes
 GH_ACCOUNT=	Cyan4973

devel/xxhash/distinfo

@@ -1,3 +1,3 @@
-TIMESTAMP = 1596954458
-SHA256 (Cyan4973-xxHash-v0.8.0_GH0.tar.gz) = 7054c3ebd169c97b64a92d7b994ab63c70dd53a06974f1f630ab782c28db0f4f
-SIZE (Cyan4973-xxHash-v0.8.0_GH0.tar.gz) = 145909
+TIMESTAMP = 1638963352
+SHA256 (Cyan4973-xxHash-v0.8.1_GH0.tar.gz) = 3bb6b7d6f30c591dd65aaaff1c8b7a5b94d81687998ca9400082c739a690436c
+SIZE (Cyan4973-xxHash-v0.8.1_GH0.tar.gz) = 171552

devel/xxhash/pkg-plist

@@ -7,7 +7,7 @@ include/xxhash.h
 lib/libxxhash.a
 lib/libxxhash.so
 lib/libxxhash.so.0
-lib/libxxhash.so.%%PORTVERSION%%
+lib/libxxhash.so.%%DISTVERSION%%
 libdata/pkgconfig/libxxhash.pc
 man/man1/xxh128sum.1.gz
 man/man1/xxh32sum.1.gz