ensure faketime can't be initialized more than once

One callsite of ftpl_init wasn't protected by the "if (!initialized)"
condition, specifically:

static void ftpl_init (void) __attribute__ ((constructor));

If another "constructor" was called before this one, and that other
constructor used time or filesystem functions, ftlp_init would be
initialized by that other constructor, and then reinitialized by the
ftpl_init constructor. At that point, confusion ensues.