Fixing multiple definitions of exitflag (szechyjs#104) (#105)

Thanks to N0YKG for the fix! I am just making the PR :)

include/dsd.h

@@ -61,7 +61,7 @@
 /*
  * global variables
  */
-int exitflag;
+extern int exitflag;
 
 
 typedef struct

src/dsd_main.c

@@ -28,6 +28,7 @@
 #include "p25p1_heuristics.h"
 #include "pa_devs.h"
 
+int exitflag = 0;
 
 int
 comp (const void *a, const void *b)