syscalls/fallocate01: Fix compilation failure.

On newer distribution the addition of _GNU_SOURCE breaks the compilation
because it exposes the FALLOC_FL_KEEP_SIZE as a macro constant which
breaks the enum defition.

So this commit adds FALLOC_FL_KEEP_SIZE to lapi/fcntl.h and removes the
enum.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>