make: Clean up build *.so if requested only

In OpenVZ -> CRIU converter I use same build engine
as in criu. But converter has own specifics such as
shipping precompiled vdso.so libraries. So when I
call for "make clean" these precompiled files get
automatically deleted, which in turn cause converter
to fail on next build attempt.

Thus lets delete only *.so targets which are declared
in makefiles.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

scripts/Makefile.build

@@ -187,6 +187,7 @@ $(obj)/$(lib-so).so: $(all-objs) $(libs-e)
 	$(Q) $(CC) -shared $(cflags-so) -o $@ $^
 
 _all += $(obj)/$(lib-so).so
+cleanup-y += $(obj)/$(lib-so).so
 endif
 
 ##
@@ -206,7 +207,6 @@ cleanup-y += $(obj)/*.o $(obj)/*.d
 cleanup-y += $(obj)/*.i $(obj)/*.s
 cleanup-y += $(obj)/*.built-in.o
 cleanup-y += $(obj)/built-in.o
-cleanup-y += $(obj)/*.so
 
 ##
 ## Predefined .PHONY targets