adapt complexity_n to leastsq inteface

include/benchmark/reporter.h

@@ -70,7 +70,7 @@ class BenchmarkReporter {
     
     // Keep track of arguments to compute asymptotic complexity
     BigO   complexity;
-    size_t complexity_n;
+    int complexity_n;
     
     // Inform print function whether the current run is a complexity report
     bool report_big_o;

src/benchmark.cc

@@ -119,7 +119,7 @@ struct ThreadStats {
     ThreadStats() : bytes_processed(0), items_processed(0), complexity_n(0) {}
     int64_t bytes_processed;
     int64_t items_processed;
-    size_t complexity_n;
+    int     complexity_n;
 };
 
 // Timer management class