Declare exit_calcurse() "noreturn".

Functions that never return should be declared "noreturn" to tell the
compiler this fact. Also, clang-analyzer will detect this attribute and
take it into account when running scan-build.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -840,7 +840,7 @@ void          todo_view_note (char *);
 void          todo_free_list (void);
 
 /* utils.c */
-void         exit_calcurse (int);
+void         exit_calcurse (int) __attribute__((__noreturn__));
 void         free_user_data (void);
 void         fatalbox (const char *);
 void         warnbox (const char *);