Fix build on uClibc Linux targets

[operutka]

Hi! Recently, I stumbled upon this problem:

error[E0425]: cannot find function `posix_fallocate` in crate `libc`
   --> /home/operutka/.cargo/registry/src/github.com-1ecc6299db9ec823/fs3-0.5.0/src/unix.rs:110:30
    |
110 |     let ret = unsafe { libc::posix_fallocate(file.as_raw_fd(), 0, len as libc::off_t) };
    |                              ^^^^^^^^^^^^^^^ help: a constant with a similar name exists: `SYS_fallocate`
    | 
   ::: /home/operutka/.cargo/registry/src/github.com-1ecc6299db9ec823/libc-0.2.79/src/unix/uclibc/arm/mod.rs:924:1

when trying to build my project for a uClibc based Linux environment. Apparently, uClibc does not have the posix_fallocate function. This PR fixes the issue.