build: fix centos rpm build

build: fix centos rpm build

When build vpp-plugins rpm package, found

/home/vpp/build-root/rpmbuild/vpp-21.10/build-root/\
install-vpp-native/vpp/lib/vpp_plugins: No such file or directory

RPM build errors:
File not found: /home/vpp/build-root/rpmbuild/../usr/lib/vpp_plugins/*

After e3cf4d0 ("build: use GNUInstallDirs install destinations")
vpp_plugins on centos src path changed from lib to lib64
Update RPM spec file accordingly.

Type: fix
Signed-off-by: Tianyu Li <[email protected]>
Change-Id: I9c4d91f97f2faa474bce28893ab763b414f759b8