sys_random: Work around an uClibc bug. Reported by akater <nuclearspace@gmail.com> via Stefan Kangas <stefan@marxist.se> in <https://debbugs.gnu.org/cgi/bugreport.cgi?bug=42236>. * lib/sys_random.in.h: On uClibc, include <stddef.h> first. * m4/sys_random_h.m4 (gl_HEADER_SYS_RANDOM): Update comment. * m4/getrandom.m4 (gl_FUNC_GETRANDOM): Likewise. * doc/glibc-headers/sys_random.texi: Mention the uClibc bug.