powerpc/pmem: Include pmem prototypes

It fixes this W=1 compile error :

../arch/powerpc/lib/pmem.c:51:6: error: no previous prototype for ‘arch_wb_cache_pmem’ [-Werror=missing-prototypes]
51 | void arch_wb_cache_pmem(void *addr, size_t size)
| ^~~~~~~~~~~~~~~~~~
../arch/powerpc/lib/pmem.c:58:6: error: no previous prototype for ‘arch_invalidate_pmem’ [-Werror=missing-prototypes]
58 | void arch_invalidate_pmem(void *addr, size_t size)
| ^~~~~~~~~~~~~~~~~~~~

Fixes: 32ce3862af3c ("powerpc/lib: Implement PMEM API")
Signed-off-by: Cédric Le Goater <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/powerpc/lib/pmem.c

···
 #include <linux/string.h>
 #include <linux/export.h>
 #include <linux/uaccess.h>
+#include <linux/libnvdimm.h>
 
 #include <asm/cacheflush.h>