Put ais_charset in its own file to make building commb tests a little easier

comm_b.c

@@ -19,6 +19,7 @@
 // along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 #include "dump1090.h"
+#include "ais_charset.h"
 
 typedef int (*CommBDecoderFn)(struct modesMessage *,bool);