core-util: include xlocale.h when using strtod_l()

core-util: include xlocale.h when using strtod_l()

Based on some googling, strtod_l() is defined in xlocale.h on BSD.
Glibc seems to define it in stdlib.h, but only if GNU extensions are
enabled (otherwise the function won't be available). So, this patch
should fix the use of strtod_l() on BSDs, but on other systems things
may or may not be still broken.

The original patch author is Jakob Fink <jfink@gmx.at>. He sent this
patch to the freebsd-gnome mailing list:
http://lists.freebsd.org/pipermail/freebsd-gnome/2015-April/032138.html

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=90285