x86: fix unconditional arch/x86/kernel/pcspeaker.o compiling

do not add the pcspkr platform device if pcspkr support is disabled.

Signed-off-by: Michael Opdenacker <michael@free-electrons.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>