milkymist: do not modify libs-softmmu

This is better and prepares for the next patch.  When we copy
libs_softmmu's value into LIBS with a := assignment, we cannot
anymore modify libs_softmmu in the Makefiles.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/display/Makefile.objs

@@ -21,7 +21,7 @@ common-obj-$(CONFIG_ZAURUS) += tc6393xb.o
 ifeq ($(CONFIG_MILKYMIST_TMU2),y)
 common-obj-y += milkymist-tmu2.o
 milkymist-tmu2.o-cflags := $(OPENGL_CFLAGS)
-libs_softmmu += $(OPENGL_LIBS)
+milkymist-tmu2.o-libs += $(OPENGL_LIBS)
 endif
 
 obj-$(CONFIG_OMAP) += omap_dss.o