build-sys: fix out-of-tree build from tarball

There are generated files in build directory too. Fixes:

CC       util.o
In file included from ../../tests/session.c:3:0:
../../gtk/spice-session.h:24:30: fatal error: spice-glib-enums.h:
No such file or directory #include "spice-glib-enums.h"