libxxhash.pc seems incorrect in libxxhash-devel-0.8.0-1

[Daisuke Fujimura]

`pkg-config libxxhash --modversion` returns empty.
(expected: 0.8.0)

$ cygcheck -cd libxxhash-devel
Cygwin Package Information
Package              Version
libxxhash-devel      0.8.0-1

$ cat /usr/lib/pkgconfig/libxxhash.pc
#   xxHash - Extremely fast hash algorithm
#   Copyright (C) 2012-2020, Yann Collet, Facebook
#   BSD 2-Clause License (https://www.opensource.org/licenses/bsd-license.php)

prefix=/usr
exec_prefix=${prefix}
includedir=${prefix}/include
libdir=${exec_prefix}/lib

Name: xxhash
Description: extremely fast hash algorithm
URL: https://www.xxhash.com/
Version:
Libs: -L${libdir} -lxxhash
Cflags: -I${includedir}