procqueue: use unsigned int for I/O frame size fields

A frame size cannot be negative, so 'unsigned' is more
suitable for frame size fields.
diff --git a/include/osmocom/gapk/procqueue.h b/include/osmocom/gapk/procqueue.h
index 50e6eb6..b4aa857 100644
--- a/include/osmocom/gapk/procqueue.h
+++ b/include/osmocom/gapk/procqueue.h
@@ -24,9 +24,9 @@
 
 struct osmo_gapk_pq_item {
 	/*! input frame size (in bytes). '0' in case of variable frames */
-	int len_in;
+	unsigned int len_in;
 	/*! output frame size (in bytes). '0' in case of variable frames */
-	int len_out;
+	unsigned int len_out;
 	/*! opaque state */
 	void *state;
 	/*! call-back for actual format conversion function