exit: annotate FUNC_ATTR_NORETURN functions #7954 (#7954)

This should fix a particular false positive from clang 5.0.0 scan-build,
which thinks that nlua_init() can continue after preserve_exit().

src/nvim/main.c

@@ -577,8 +577,9 @@ int main(int argc, char **argv)
   return 0;
 }
 
-/* Exit properly */
+/// Exit properly
 void getout(int exitval)
+  FUNC_ATTR_NORETURN
 {
   tabpage_T   *tp, *next_tp;