spec: use macro for sysusers.d path

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>

redhat/rdma-core.spec

diff --git a/redhat/rdma-core.spec b/redhat/rdma-core.spec
index 6cc9e1f..8b605d2 100644
--- a/redhat/rdma-core.spec
+++ b/redhat/rdma-core.spec
@@ -613,7 +613,7 @@
 %{_libdir}/libmlx4.so.*
 %config(noreplace) %{_sysconfdir}/libibverbs.d/*.driver
 %doc %{_docdir}/%{name}/libibverbs.md
-%{_prefix}/lib/sysusers.d/rdma.conf
+%{_sysusersdir}/rdma.conf
 
 %files -n libibverbs-utils
 %{_bindir}/ibv_*