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>
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 2d13e68..e87503e 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -674,6 +674,7 @@
 
 		case 'x':
 			flags |= SMALL;
+		/* fallthrough */
 		case 'X':
 			base = 16;
 			break;
@@ -681,8 +682,10 @@
 		case 'd':
 			if (fmt[1] == 'E')
 				flags |= ERRSTR;
+		/* fallthrough */
 		case 'i':
 			flags |= SIGN;
+		/* fallthrough */
 		case 'u':
 			break;