Handle fallthrough with attribute(noreturn) for consistancy and compatiblity with older GCC

qubes-rpc/gui-fatal.h

@@ -1,3 +1,3 @@
-void gui_fatal(const char *fmt, ...);
+void gui_fatal(const char *fmt, ...) __attribute__((noreturn));
 void gui_nonfatal(const char *fmt, ...);
-void qfile_gui_fatal(const char *fmt, va_list args);
+void qfile_gui_fatal(const char *fmt, va_list args) __attribute__((noreturn));