Don't include fmt/os.h in the module if it is disabled via FMT_OS #5285

Job	Run time
mingw (mingw64)	4m 17s
build (windows-2019, Win32, v142, 17, Debug)	1m 48s
mingw (ucrt64)	4m 13s
build (windows-2019, Win32, v142, 17, Release)	2m 37s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	1m 5s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	1m 47s
build (windows-2019, x64, v140, 14, Debug)	1m 6s
build (windows-2019, x64, v140, 14, Release)	2m 17s
build (windows-2019, x64, v141, 17, Debug)	1m 48s
build (windows-2019, x64, v141, 17, Release)	2m 17s
build (windows-2019, x64, v142, 17, Debug)	1m 5s
build (windows-2019, x64, v142, 17, Release)	2m 21s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	1m 8s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	1m 57s
build (windows-2019, x64, v142, 20, Debug)	2m 4s
build (windows-2019, x64, v142, 20, Release)	2m 34s
build (windows-2022, x64, v143, Debug, 20)	1m 22s
	35m 46s

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