Update after gnulib changed.
* gettext-runtime/libasprintf/autosprintf.cc: Include <config.h>. This is
needed because of these new gnulib module dependencies:
vasnprintf -> free-posix -> stdlib -> unistd,
and the generated unistd.h gets included indirectly by <string> on OpenBSD 6.7
and gives a #error "Please include config.h first."
