[tlbuild] build failure: texk/dvisvgm/dvisvgm-0.8.7/src/FileSystem.cpp includes <unistd.h> only for GCC, but it is needed on all posix systems

[Karl Berry]

    texk/dvisvgm/dvisvgm-0.8.7/src/FileSystem.cpp has:

    #ifdef __GNUC__
    #include <unistd.h>
    #endif

    after removing "#ifdef __GNUC__" and "#endif", compilation proceeds. 

I removed the #ifdef.  No conditional should be necessary, <unistd.h>
is ubiquitious these days.  Much recent GNU software unconditionally
includes it, compiled on far more platforms than TL.

Thanks,
Karl