Bump gimlet-seq stacksize (#1750)

I'm seeing stack overflow for `gimlet-seq` on the `gimlet-c-dev` image.
Running with a boosted stack size, I see a `stackmargin` size of 1728,
which is above the limit of 1600. Bisecting, this appears to have been
introduced in #1730; I'm guessing it's the nested
`ServerImpl::handle_notifications → VCore::handle_notifications`
dispatch.

app/gimlet/base.toml

@@ -163,7 +163,7 @@ name = "drv-gimlet-seq-server"
 features = ["h753"]
 priority = 4
 max-sizes = {flash = 131072, ram = 16384 }
-stacksize = 1600
+stacksize = 2600
 start = true
 task-slots = ["sys", "i2c_driver", {spi_driver = "spi2_driver"}, "hf", "jefe", "packrat"]
 notifications = ["timer", "vcore"]