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>

arch/powerpc/platforms/pseries/plpar_wrappers.h

 #ifndef _PSERIES_PLPAR_WRAPPERS_H
 #define _PSERIES_PLPAR_WRAPPERS_H
 
+#include <linux/string.h>
+
 #include <asm/hvcall.h>
+#include <asm/paca.h>
 #include <asm/page.h>
 
 /* Get state of physical CPU from query_cpu_stopped */