powerpc: fix two implicit header uses in pseries/plpar_wrappers.h

powerpc: fix two implicit header uses in pseries/plpar_wrappers.h

Removing the implicit presence of module.h from almost everywhere
will reveal this implicit usage of paca.h and string.h headers as
follows:

arch/powerpc/platforms/pseries/plpar_wrappers.h:22: error: implicit declaration of function 'get_lppaca'
arch/powerpc/platforms/pseries/plpar_wrappers.h:208: error: implicit declaration of function 'memcpy'

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