grub-core/Makefile.am: Make path to extra_deps.lst relative to $(top_srcdir)/grub...

grub-core/Makefile.am: Make path to extra_deps.lst relative to $(top_srcdir)/grub-core

The commit 154dcb1ae (build: Allow explicit module dependencies) broke
out of tree builds by introducing the extra_deps.lst file into the
source tree but referencing it just by name in grub-core/Makefile.am.
Fix it by adding $(top_srcdir)/grub-core to the path.

Fixes: 154dcb1ae (build: Allow explicit module dependencies)
Signed-off-by: Mate Kukri <mate.kukri@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>