Fix 64-bit benchmark build

The * flag to printf() wants an int instead of size_t, and these are
distinct types on 64-bit.  To accomodate this, make the name column
width helpers return int.

In theory this truncates things, but in practice this only matters if
you have a benchmark with more than INT_MAX characters in its name (in
which case you have bigger problems).

Change-Id: I3338948c25a3a8d84f1ead2f5b457c05da8a01cf
Signed-off-by: Greg Hackmann <ghackmann@google.com>

benchmarks/Benchmark.cpp

@@ -44,8 +44,8 @@ std::vector<Benchmark*>& Benchmark::List() {
   return list;
 }
 
-size_t Benchmark::MaxNameColumnWidth() {
-  size_t max = 20;
+int Benchmark::MaxNameColumnWidth() {
+  int max = 20;
   for (auto& benchmark : List()) {
     max = std::max(max, benchmark->NameColumnWidth());
   }