Add additional instantiations in Instantiations.cpp

Add additional instantiations in Instantiations.cpp

Summary:
Histogram::getPercentileBucketIdx() and getPercentileEstimate() are
implemented using template methods of HistogramBuckets.

This updates Instantiations.cpp to also instantiate the versions of
these template methods that are required by Histogram<int64_t>.  Without
this, anyone using these methods was required to also include
Histogram-defs.h, or otherwise they may get link errors.  This makes it
so they no longer need to include Histogram-defs.h

The C++ syntax required is pretty horrendous.  We could potentially
avoid this by refactoring the code so that these methods are no longer
templates (and require that the Bucket class provide methods for this
purpose).  At the moment adding instantiations seems simpler, and
doesn't change any public APIs.

Test Plan:
Successfully linked code using
Histogram<int64_t>::getPercentileEstimate() without including
Histogram-defs.h

Reviewed By: ldbrandy@fb.com, meyering@fb.com

Subscribers: trunkagent, doug, net-systems@, exa, folly-diffs@

FB internal diff: D1786346

Signature: t1:1786346:1421429734:da17b7ea326c64a1e158fec3b87dc49c3db3d848