Fix JET warning in Base.sendfile (#60466)
Help JET (and humans?) understand that src_file and dst_file won't be
used without being defined.
Fixes this JET report:
```
│┌ cp(src::AbstractString, dst::String) @ Base.Filesystem ./file.jl:404
││┌ cp(src::AbstractString, dst::String; force::Bool, follow_symlinks::Bool) @ Base.Filesystem ./file.jl:410
│││┌ kwcall(::@NamedTuple{force::Bool, follow_symlinks::Bool}, ::typeof(Base.Filesystem.cptree), src::String, dst::String) @ Base.Filesystem ./file.jl:366
││││┌ cptree(src::String, dst::String; force::Bool, follow_symlinks::Bool) @ Base.Filesystem ./file.jl:379
│││││┌ sendfile(src::String, dst::String) @ Base.Filesystem ./file.jl:1282
││││││ local variable `src_file` may be undefined: src_file::Base.Filesystem.File
│││││└────────────────────
│││││┌ sendfile(src::String, dst::String) @ Base.Filesystem ./file.jl:1283
││││││ local variable `src_file` may be undefined: src_file::Base.Filesystem.File
│││││└────────────────────
│││││┌ sendfile(src::String, dst::String) @ Base.Filesystem ./file.jl:1285
││││││ local variable `dst_file` may be undefined: dst_file::Base.Filesystem.File
│││││└────────────────────
│││││┌ sendfile(src::String, dst::String) @ Base.Filesystem ./file.jl:1286
││││││ local variable `dst_file` may be undefined: dst_file::Base.Filesystem.File
│││││└────────────────────
```
