[POWERPC] Export copy_page() on 32bit

Export copy_page() on 32-bit powerpc; unionfs needs it.

Unionfs already builds as a module on 64bit powerpc, so the export is
placed within an existing CONFIG_PPC32 #ifdef.
Signed-off-by: Joseph Fannin <jfannin@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/ppc_ksyms.c

@@ -59,6 +59,7 @@ extern void single_step_exception(struct pt_regs *regs);
 extern int sys_sigreturn(struct pt_regs *regs);
 
 EXPORT_SYMBOL(clear_pages);
+EXPORT_SYMBOL(copy_page);
 EXPORT_SYMBOL(ISA_DMA_THRESHOLD);
 EXPORT_SYMBOL(DMA_MODE_READ);
 EXPORT_SYMBOL(DMA_MODE_WRITE);