sungem: fix compile failure caused by trivial #include consolidation

Only Sparc and PPC actually have the asm/prom.h include and as such they
can't be moved outside of the ifdefs.

Reported-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>