x86/boot: Handle relative CONFIG_EFI_SBAT_FILE file paths

CONFIG_EFI_SBAT_FILE can be a relative path. When compiling using a different
output directory (O=) the build currently fails because it can't find the
filename set in CONFIG_EFI_SBAT_FILE:

arch/x86/boot/compressed/sbat.S: Assembler messages:
arch/x86/boot/compressed/sbat.S:6: Error: file not found: kernel.sbat

Add $(srctree) as include dir for sbat.o.

[ bp: Massage commit message. ]

Fixes: 61b57d35396a ("x86/efi: Implement support for embedding SBAT data for x86")
Signed-off-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: <stable@kernel.org>
Link: https://patch.msgid.link/f4eda155b0cef91d4d316b4e92f5771cb0aa7187.1772047658.git.jstancek@redhat.com