pod: Fix spa_pod_builder_control() return type

Fixes the return type of spa_pod_builder_control() from uint32_t to int.

Since the function returns the int returned by spa_pod_builder_raw, the return type of the function should also be an int.

This shouldn't break API, since the implicit conversion from int to uint32_t simply moves outside of the function when people assign this to a uint32_t.
And most of the C code will be using a convenience function/macro anyways.