test/unitest_bluestore_types: fix a compilation error

The test was failing to compile because of a missing
'include' directive.
Problem was introduced in commit 2e73c87

Signed-off-by: Ronen Friedman <[email protected]>

Author: ronen-fr

src/test/objectstore/test_bluestore_types.cc

@@ -16,6 +16,7 @@
 #include "os/bluestore/Writer.h"
 #include "common/pretty_binary.h"
 
+#include <bitset>
 #include <sstream>
 
 #define _STR(x) #x