LinuxKPI: add module dependency on firmware(9)

In a6c2507d1baedb183268e31bc6b6f659a9529904 support for LinuxKPI
firmware loading was added. Record the dependency on firmware(9)
as otherwise (if built as module) linuxkpi will no longer load.

Reported-by: tijl
MFC after: 1 day
X-MFC-with: a6c2507d1baedb183268e31bc6b6f659a9529904
Sponsored-by: The FreeBSD Foundation