linker: Add MODINFOMD_SPLASH type

This mod type and structure holds the splash screen data and metadata.

Differential Revision: https://reviews.freebsd.org/D45930
Reviewed by: tsoome
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 966e53a4e563da36d62edf57c2b0007921fec8dd)

splash: Add sys/types.h

To fix make test-includes

Fixes: 966e53a4e563 ("linker: Add MODINFOMD_SPLASH type")
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit e4fc2fd62a5e4afebfc466868eb3b17f9ba0ad57)