seabios: update submodule to 1.15.0

Update seabios to the final release.  No code changes
compared to the snapshot merged a few weeks ago.

shortlog 64f37cc530f1..rel-1.15.0
---------------------------------

Kevin O'Connor (1):
      docs: Note v1.15.0 release

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

diff --git a/roms/seabios b/roms/seabios
index 64f37cc530f144e53c190c9e8209a51b58fd5c43..2dd4b9b3f84019668719344b40dba79d681be41c 160000 (submodule)
--- a/roms/seabios
+++ b/roms/seabios
@@ -1 +1 @@
-Subproject commit 64f37cc530f144e53c190c9e8209a51b58fd5c43
+Subproject commit 2dd4b9b3f84019668719344b40dba79d681be41c