i965/gen7: Add missing ! to brw->gs.prog_active assertion.

A typo in commit c173541d97 accidentally removed the !.
It's supposed to assert that there is _not_ an active GS program.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38762

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/gen7_urb.c

@@ -78,7 +78,7 @@ upload_urb(struct brw_context *brw)
    assert(brw->urb.nr_vs_entries % 8 == 0);
    assert(brw->urb.nr_gs_entries % 8 == 0);
    /* GS requirement */
-   assert(brw->gs.prog_active);
+   assert(!brw->gs.prog_active);
 
    BEGIN_BATCH(2);
    OUT_BATCH(_3DSTATE_PUSH_CONSTANT_ALLOC_VS << 16 | (2 - 2));