libfmt: add missing copyright notice to nan64.c

nan64.c was part of change #1 in this repository and
did not have a notice at that point (September 2003).
An earlier copy of libfmt, dated January 14-15, 2003,
makes it clear that nan64.c was extracted from fltfmt.c
and strtod.c, so use the same notice they use.

src/lib9/fmt/nan64.c

@@ -1,3 +1,5 @@
+/* Copyright (c) 2002-2006 Lucent Technologies; see LICENSE */
+
 /*
  * 64-bit IEEE not-a-number routines.
  * This is big/little-endian portable assuming that