Take fcvt.c from musl instead of our modified copy

The only change in our fcvt.c was to circumvent a conflict with glibc's
headers, but this is irrelevant because we no longer use glibc headers.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>