externals: Update fmt to 6201052

Previously, we'd get warnings like:

"
c:\projects\yuzu\externals\fmt\include\fmt\format.h(2868): warning
C4127: conditional expression is constant
[C:\projects\yuzu\msvc_build\externals\dynarmic\src\dynarmic.vcxproj]
"

spamming the build output when compiling on Windows. This updates fmt to
include the upstreamed fix that silences this warning.