Rename spawn-polkit-agent.c to just polkit-agent.c

The functions in the file were *not* using the spawn prefix,
and the prefix seems completely unnecessary.

src/systemctl/systemctl-util.c

@@ -19,11 +19,11 @@
 #include "macro.h"
 #include "path-util.h"
 #include "pidref.h"
+#include "polkit-agent.h"
 #include "process-util.h"
 #include "reboot-util.h"
 #include "set.h"
 #include "spawn-ask-password-agent.h"
-#include "spawn-polkit-agent.h"
 #include "stat-util.h"
 #include "systemctl-util.h"
 #include "systemctl.h"