In fmt master, basic_format_string has moved from base.h to xchar.h

Took place in this commit

This change causes compile time errors due to:-

1. Using SPDLOG_FMT_EXTERNAL and using fmt master
2. spdlog/common needing basic_format_string without SPDLOG_USE_STD_FORMAT https://github.com/gabime/spdlog/blob/e593f6695c6065e6b345fe2862f04a519ed484e0/include/spdlog/common.h#L367-L370
3. xchar.h not being included due to absence of the specified definitions https://github.com/gabime/spdlog/blob/e593f6695c6065e6b345fe2862f04a519ed484e0/include/spdlog/common.h#L56-L58