Fixed a compile error in util/options_builder.cc

Summary:
Fixed the following compile error by replacing pow by shift, as it computes
power of 2.

util/options_builder.cc:133:14: error: no member named 'pow' in namespace 'std'
        std::pow(2, std::max(0, std::min(3, level0_stop_writes_trigger -
        ~~~~~^
1 error generated.
make: *** [util/options_builder.o] Error 1

Test Plan: make success in mac and linux

Reviewers: ljin, igor, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20475

util/options_builder.cc

@@ -130,8 +130,8 @@ void OptimizeForLevel(int read_amplification_threshold,
 
     // Try to enlarge the buffer up to 1GB, if still have sufficient headroom.
     file_num_buffer *=
-        std::pow(2, std::max(0, std::min(3, level0_stop_writes_trigger -
+        1 << std::max(0, std::min(3, level0_stop_writes_trigger -
-                                                file_num_buffer - 2)));
+                                       file_num_buffer - 2));
 
     options->level0_stop_writes_trigger = level0_stop_writes_trigger;
     options->level0_slowdown_writes_trigger = level0_stop_writes_trigger - 2;