Build warnings (-Wsign-conversion) under Clang

[Karsten1987]

Originally reported in here: ros2/spdlog_vendor#4

There is a quite a few warnings under clang which ideally should be addressed. Pretty much all in the style of converting int to size_t:

In file included from /opt/ros/master/build/spdlog_vendor/spdlog-1.5.0-prefix/src/spdlog-1.5.0/src/fmt.cpp:10:
/opt/ros/master/build/spdlog_vendor/spdlog-1.5.0-prefix/src/spdlog-1.5.0/include/spdlog/fmt/bundled/format-inl.h:484:48: warning: implicit conversion changes signedness: 'int' to 'unsigned long long' [-Wsign-conversion]
          (min_exponent + fp::significand_size - 1) * one_over_log2_10 +`

[vitaut]

Fixed in 96c68af.