mon/PGMonitor: mark dump_object_stat_sum() as static

so we can test this method without create an instance of PGMonitor.

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit af3d3e2)

Conflicts:
	src/mon/PGMonitor.cc ("const" suffix of function definition was removed
            in master, but in hammer it wasn't there, so no change necessary)
	src/mon/PGMonitor.h (added static prefix to function declaration; did
            not remove "const" suffix because it wasn't there in hammer)

src/mon/PGMonitor.h

@@ -145,7 +145,7 @@ class PGMonitor : public PaxosService {
 			  int threshold,
 			  vector<string>& args) const;
 
-  void dump_object_stat_sum(TextTable &tbl, Formatter *f,
+  static void dump_object_stat_sum(TextTable &tbl, Formatter *f,
 			    object_stat_sum_t &sum,
 			    uint64_t avail,
 			    float raw_used_rate,