ensure statvfs64 is declared

[Im-0xea]

I was testing btop on risc-v with musl, and I got build errors because statvfs64 is not defined, my first instinct was to just step down to statvfs(not 64) if the LFS defines are not set, #555, because of a bit of a miss understanding of mine of how they work, sorry for that >.<, anyhow this should ensure that under musl libc and glibc statvfs64 is defined.

[imwints]

This is related to #508 and will be closed in #510.

From the Musl FAQ:

-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE have nothing to do with supporting large files properly but with exposing the idiotic legacy interfaces with 64 on the end of their names (like open64, lseek64, etc.). only -D_FILE_OFFSET_BITS=64 should ever be used, anywhere, even on glibc.