powerpc: do not build ppc_save_regs.o always

The Fixes commit below tried to add CONFIG_PPC_BOOK3S to one of the
conditions to enable the build of ppc_save_regs.o. But it failed to do
so, in fact. The commit omitted to add a dollar sign.

Therefore, ppc_save_regs.o is built always these days (as
"(CONFIG_PPC_BOOK3S)" is never an empty string).

Fix this by adding the missing dollar sign.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Fixes: fc2a5a61 ("powerpc/64s: ppc_save_regs is now needed for all 64s builds")
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250417105305.397128-1-jirislaby@kernel.org