shared: add %g, %G specifiers for group / gid (#10368)

diff --git a/src/tmpfiles/tmpfiles.c b/src/tmpfiles/tmpfiles.c
index 1b3cb5f..d7e02c4 100644
--- a/src/tmpfiles/tmpfiles.c
+++ b/src/tmpfiles/tmpfiles.c
@@ -174,6 +174,8 @@
         { 'H', specifier_host_name,       NULL },
         { 'v', specifier_kernel_release,  NULL },
 
+        { 'g', specifier_group_name,      NULL },
+        { 'G', specifier_group_id,        NULL },
         { 'U', specifier_user_id,         NULL },
         { 'u', specifier_user_name,       NULL },
         { 'h', specifier_user_home,       NULL },