Don't include fmt/os.h in the module if it is disabled via FMT_OS #5284

Job	Run time
mingw (mingw64)	4m 24s
build (windows-2019, Win32, v142, 17, Debug)	1m 26s
build (windows-2019, Win32, v142, 17, Release)	2m 36s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	1m 24s
mingw (ucrt64)	4m 35s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	2m 7s
build (windows-2019, x64, v140, 14, Debug)	1m 27s
build (windows-2019, x64, v140, 14, Release)	1m 43s
build (windows-2019, x64, v141, 17, Debug)	1m 36s
build (windows-2019, x64, v141, 17, Release)	2m 14s
build (windows-2019, x64, v142, 17, Debug)	1m 33s
build (windows-2019, x64, v142, 17, Release)	2m 16s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	1m 26s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	1m 55s
build (windows-2019, x64, v142, 20, Debug)	1m 33s
build (windows-2019, x64, v142, 20, Release)	3m 0s
build (windows-2022, x64, v143, Debug, 20)	2m 5s
	37m 20s

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