Clean up linkage of struct Modes to actually make sense.

(how did this work before? But it's been unchanged since at least
2013..)

Maybe fixes #65

dump1090.c

@@ -51,6 +51,8 @@
 
 #include <stdarg.h>
 
+struct _Modes Modes;
+
 //
 // ============================= Utility functions ==========================
 //

dump1090.h

@@ -295,7 +295,7 @@ struct mag_buf {
 };
 
 // Program global state
-struct {                             // Internal state
+struct _Modes {                             // Internal state
     pthread_t       reader_thread;
 
     pthread_mutex_t data_mutex;      // Mutex to synchronize buffer access
@@ -397,7 +397,9 @@ struct {                             // Internal state
     int stats_latest_1min;
     struct stats stats_5min;
     struct stats stats_15min;
-} Modes;
+};
+
+extern struct _Modes Modes;
 
 // The struct we use to store information about a decoded message.
 struct modesMessage {

faup1090.c

@@ -49,6 +49,8 @@
 
 #include "dump1090.h"
 
+struct _Modes Modes;
+
 #include <stdarg.h>
 
 void receiverPositionChanged(float lat, float lon, float alt)

view1090.c

@@ -28,6 +28,9 @@
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 //
 #include "dump1090.h"
+
+struct _Modes Modes;
+
 //
 // ============================= Utility functions ==========================
 //