build: use -$(CONFIG_SECCOMP) instead of ifeq

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

Makefile.objs

@@ -80,11 +80,7 @@ common-obj-$(CONFIG_SLIRP) += slirp/
 
 common-obj-y += backends/
 
-######################################################################
-# libseccomp
-ifeq ($(CONFIG_SECCOMP),y)
-common-obj-y += qemu-seccomp.o
-endif
+common-obj-$(CONFIG_SECCOMP) += qemu-seccomp.o
 
 common-obj-$(CONFIG_SMARTCARD_NSS) += $(libcacard-y)