lib: missing fallthrough comment in vsnprintf_internal()

When a case statement intentionally falls through we should add a comment.
Cf. -Wimplicit-fallthrough

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

lib/vsprintf.c

@@ -674,6 +674,7 @@ repeat:
 
 		case 'x':
 			flags |= SMALL;
+		/* fallthrough */
 		case 'X':
 			base = 16;
 			break;
@@ -681,8 +682,10 @@ repeat:
 		case 'd':
 			if (fmt[1] == 'E')
 				flags |= ERRSTR;
+		/* fallthrough */
 		case 'i':
 			flags |= SIGN;
+		/* fallthrough */
 		case 'u':
 			break;