selftests/powerpc: Fix missing clean of pmu/lib.o

The tm-resched-dscr test links against pmu/lib.o, but we don't have a
rule to clean pmu/lib.o. This can lead to a build break if you build
for big endian and then little, or vice versa.

Fix it by making tm-resched-dscr depend on pmu/lib.c, causing the code
to be built directly in, meaning no .o is generated.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>