!8 fix build failure that unitdir cannot be identified

From: @zmr_2020 
Reviewed-by: @biannm 
Signed-off-by: @biannm

alertmanager.spec

@@ -2,7 +2,7 @@
 
 Name:    alertmanager
 Version: 0.21.0
-Release: 4
+Release: 5
 Summary: Prometheus Alertmanager.
 License: ASL 2.0
 URL:     https://github.com/prometheus/%{name}
@@ -14,6 +14,7 @@ Source2: %{name}.default
 Source3: %{name}.yml
 
 BuildRequires: golang
+BuildRequires: systemd
 
 %{?systemd_requires}
 Requires(pre): shadow-utils
@@ -65,6 +66,9 @@ exit 0
 %dir %attr(755, prometheus, prometheus)%{_sharedstatedir}/prometheus
 
 %changelog
+* Thu Mar 2 2023 zhangjian <zhang_jian7@hoperun.com> - 0.21.0-5
+- fix build failure that unitdir cannot be identified
+
 * Sat Jan 29 2022 yangzhao <yangzhao1@kylinos.cn> - 0.21.0-4
 - Build from source code
 
@@ -74,4 +78,4 @@ exit 0
 - DESC: solve empty postun transaction issue.
 
 * Tue Aug 11 2020 houjian <houjian@kylinos.cn> - 0.21.0-2
-- Package init
+- Package init