Fix xregex2.h:__restrict_arr

[DJ Delorie]

> The reason the weird new warning is coming up is because GLIBC's
> sys/cdefs.h always uses __restrict if GCC supports it, it fails to
> check STDC_VERSION like xregex2.h properly does.  Libiberty is
> compiled with -pedantic, thus the C89 warning.

I think I had a patch to blindly do "undef __restrict" before the
define sitting in my sandbox for a while, too, but I prefer a real
solution ;-)

Thanks!