JSONReporter: RoundDouble(): use std::lround() to round double to int
JSONReporter: RoundDouble(): use std::lround() to round double to int

From clang-tidy bugprone-incorrect-roundings check:
> casting (double + 0.5) to integer leads to incorrect rounding; consider using lround (#include <cmath>) instead

google/benchmark,google/benchmark,mockingbirdnest/benchmark,google/benchmark,mockingbirdnest/benchmark,mockingbirdnest/benchmark,mockingbirdnest/benchmark