update m4/extern-inline.m4 to upstream version to fix compilation on FreeBSD

update m4/extern-inline.m4 to upstream version to fix compilation on FreeBSD

In file included from util/grub-mkimage.c:54:0:
./grub-core/gnulib/argp.h:627:49: error: '__sbistype' is static but
used in inline function '_option_is_short' which is not static
[-Werror] cc1: all warnings being treated as errors gmake[2]: ***
[util/grub_mkimage-grub-mkimage.o] Error 1

Update m4/extern-inline.m4 to current upstream gnulib version that
contains fix for this (commit b9bfe78424b871f5b92e5ee9e7d21ef951a6801d).

Reported-By: Beeblebrox <zaphod@berentweb.com>