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

paging.c:259: warning: implicit declaration of function ‘trx_is_usable’

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;