8196988: Resolve disabled warnings for libjimage

Fixed warnings related to implicit-fallthrough gcc option

Reviewed-by: psandoz, mchung

src/java.base/share/native/libjimage/imageDecompressor.cpp

@@ -322,6 +322,7 @@ void SharedStringDecompressor::decompress_resource(u1* data,
             {
                 i++;
             }
+            /* fall through */
             default:
             {
                 *uncompressed_resource = tag;