hw/core: nmi.c can be compiled as common-obj nowadays

The target-specific code in nmi.c has been removed with this commit:

	commit f7e981f2
	nmi: remove x86 specific nmi handling

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>