powerpc: Fix up implicit stat.h users

They get it via module.h (via device.h) but we want to clean that up.
When we do, we'll get things like:

ibmebus.c:314: error: 'S_IWUSR' undeclared here (not in a function)
vio.c:972: error: 'S_IWUSR' undeclared here (not in a function)

so add in the stat header it is using explicitly in advance.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>