Fix conflicting types of virInitctlSetRunLevel

aebbcdd3 didn't change the non-linux definition of the function,
breaking the build on FreeBSD:

../../src/util/virinitctl.c:164: error: conflicting types for
'virInitctlSetRunLevel'
../../src/util/virinitctl.h:40: error: previous declaration of
'virInitctlSetRunLevel' was here

src/util/virinitctl.c

@@ -159,8 +159,7 @@ cleanup:
     return ret;
 }
 #else
-int virInitctlSetRunLevel(virInitctlRunLevel level ATTRIBUTE_UNUSED,
-                          const char *vroot ATTRIBUTE_UNUSED)
+int virInitctlSetRunLevel(virInitctlRunLevel level ATTRIBUTE_UNUSED)
 {
     virReportUnsupportedError();
     return -1;