powerpc/lib: only build ldstfp.o when CONFIG_PPC_FPU is set

The entire code in ldstfp.o is enclosed into #ifdef CONFIG_PPC_FPU,
so there is no point in building it when this config is not selected.

Fixes: cd64d169

 ("powerpc: mtmsrd not defined")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>