cxl: Compile with -Werror

It's a good idea, and it brings us in line with the rest of arch/powerpc.
Signed-off-by: NDaniel Axtens <dja@axtens.net>
Acked-by: NMichael Neuling <mikey@neuling.org>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

drivers/misc/cxl/Makefile

+ccflags-y := -Werror
+
 cxl-y				+= main.o file.o irq.o fault.o native.o
 cxl-y				+= context.o sysfs.o debugfs.o pci.o trace.o
 cxl-y				+= vphb.o api.o