Ensure gc_type is StgWord8.

Again, the range of gc_type is actually 1-3, which is technically
outside the range of rtsBool.
Signed-off-by: Austin Seipp <aseipp@pobox.com>

rts/Schedule.c

@@ -1437,7 +1437,7 @@ scheduleDoGC (Capability **pcap, Task *task USED_IF_THREADS,
     rtsBool heap_census;
     nat collect_gen;
 #ifdef THREADED_RTS
-    rtsBool gc_type;
+    StgWord8 gc_type;
     nat i, sync;
     StgTSO *tso;
 #endif