ppc: Remove duplicate export of get_wchan

The arch/powerpc version of process.c exports get_wchan itself.  When
I moved ARCH=ppc over to using arch/powerpc/kernel/process.c the
get_wchan export in arch/ppc/kernel/ppc_ksyms.c became redundant, so
remove it.

Signed-off-by: Paul Mackerras <paulus@samba.org>
(cherry picked from 9871166ad692121d6b944159ef3f053570158ea8 commit)