[paging] Include chan_alloc.h to silence a compiler warning

paging.c:259: warning: implicit declaration of function ‘trx_is_usable’
diff --git a/openbsc/src/paging.c b/openbsc/src/paging.c
index 9a4f365..314d3d1 100644
--- a/openbsc/src/paging.c
+++ b/openbsc/src/paging.c
@@ -45,6 +45,7 @@
 #include <openbsc/signal.h>
 #include <openbsc/abis_rsl.h>
 #include <openbsc/gsm_data.h>
+#include <openbsc/chan_alloc.h>
 
 void *tall_paging_ctx;