swrast: use STATIC_ASSERT to make sure texfetch table isn't missing entries

When new MESA_FORMAT_x enums are added we need to add a new entry in
the table of texture fetch functions.  In the past this has been
missed if swrast isn't actually tested.  Using a static assertion
should help with that.

src/mesa/swrast/s_texfetch.c

@@ -1282,7 +1282,8 @@ _mesa_get_texel_fetch_func(gl_format format, GLuint dims)
    }
 #endif
 
-   assert(Elements(texfetch_funcs) == MESA_FORMAT_COUNT);
+   STATIC_ASSERT(Elements(texfetch_funcs) == MESA_FORMAT_COUNT);
+
    assert(format < MESA_FORMAT_COUNT);
 
    switch (dims) {