CA-145257: Set the right month for host_crashdump.

Date.of_float increments tm_mon by 1.So, we need to pass tm_mon value
decremented by 1.

Signed-off-by: Ravi Pandey <ravi.pandey@citrix.com>
I think this looks a bit ugly now with that extra `Unix.`. I think what would be better is for the scanf to not capture the month as `tm_mon` since it clearly is not that (since tm_mon is month -1). Then you could have something like:

```diff
try Scanf.sscanf filename "%04d%02d%02d-%02d%02d%02d-UTC"
-				(fun tm_year tm_mon tm_mday tm_hour tm_min tm_sec ->
+				(fun tm_year mon tm_mday tm_hour tm_min tm_sec ->
 					fst (Unix.mktime
-						{ Unix.tm_year=tm_year-1900; tm_mon; tm_mday;
+						{ Unix.tm_year=tm_year-1900; tm_mon=mon-1; tm_mday;
```