build: Include grub-core/extra_deps.lst in dist

build: Include grub-core/extra_deps.lst in dist

Fixes build failure due to the extra_deps.lst file not existing in the
tarball. Found while trying to package GRUB 2.12 for Gentoo.

  make[3]: *** No rule to make target '/var/tmp/portage/sys-boot/grub-2.12/work/grub-2.12/grub-core/extra_deps.lst', needed by 'syminfo.lst'.  Stop.

Fixes: 89fbe0cac (grub-core/Makefile.am: Make path to extra_deps.lst relative to $(top_srcdir)/grub-core)
Fixes: 154dcb1ae (build: Allow explicit module dependencies)
Signed-off-by: Oskari Pirhonen <xxc3ncoredxx@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>