Remove the fileapi.h include in os-inl.h (#3444)

* Replaced fileapi.h include with windows.h, as instructed in https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-flushfilebuffers
Otherwise this causes compilation error on older sdks.

* Removed the fileapi.h include entirely, since windows.h is already included before

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Co-authored-by: MasonAngelio <MasonAngelio>

Author: MasonAngelio

include/spdlog/details/os-inl.h

@@ -23,7 +23,6 @@
 
 #ifdef _WIN32
     #include <spdlog/details/windows_include.h>
-    #include <fileapi.h>  // for FlushFileBuffers
     #include <io.h>       // for _get_osfhandle, _isatty, _fileno
     #include <process.h>  // for _get_pid