exa: promise not to touch the data when swapping pointers

exa/exa.c:525:10: warning: initialization discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
     swap(pExaGC, pGC, funcs);
          ^

Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

exa/exa_priv.h

@@ -244,7 +244,7 @@ extern DevPrivateKeyRec exaScreenPrivateKeyRec;
 }
 #else
 #define swap(priv, real, mem) {\
-    void *tmp = priv->Saved##mem; \
+    const void *tmp = priv->Saved##mem; \
     priv->Saved##mem = real->mem; \
     real->mem = tmp; \
 }