grub2: add PKG_FIXUP:=autoreconf

The host-side build of grub2 requires this sometimes.
This will re-generate the ./configure script from configure.ac.
I don't know the conditions of how this reproduces, it just
sometimes appears, and sometimes doesn't.

Build error
```
<lede-dir>/build_dir/target-x86_64_musl-1.1.15_yogi/host/grub-2.02~beta2/build-aux/missing: line 81: aclocal-1.14: command not found
WARNING: 'aclocal-1.14' is missing on your system.
         You should only need it if you modified 'acinclude.m4' or
         'configure.ac' or m4 files included by 'configure.ac'.
         The 'aclocal' program is part of the GNU Automake package:
         <http://www.gnu.org/software/automake>
         It also requires GNU Autoconf, GNU m4 and Perl in order to run:
         <http://www.gnu.org/software/autoconf>
         <http://www.gnu.org/software/m4/>
         <http://www.perl.org/

>
Makefile:3962: recipe for target 'aclocal.m4' failed
```

Adding PKG_FIXUP adds sanity (i.e. autoreconf is used for host & target
builds) over just using HOST_FIXUP.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>