Fix compilation on musl

musl doesn't have O_FSYNC defined and O_SYNC is used in place. [0] O_SYNC in glibc also returns the same value as O_FSYNC. [1]

[0] https://git.musl-libc.org/cgit/musl/commit/?id=c1f4ed150137d793c9d07356305a89e8785e7e02 [1] https://www.gnu.org/software/libc/manual/html_node/Operating-Modes.html#index-O_005fSYNC