Include `fsp_header.h` from vendorcode for dynamic FSP_INFO_HEADER selection. BUG=b:242829490 TEST=google/rex0 builds successfully with 64-bit FSP. Change-Id: If165e0517752f320d898cf82f298aa9f5699ae86 Signed-off-by: Subrata Banik <subratabanik@google.com> Reviewed-on: https://review.coreboot.org/c/coreboot/+/81624 Reviewed-by: Arthur Heymans <arthur@aheymans.xyz> Tested-by: build bot (Jenkins) <no-reply@coreboot.org>