BUG#28125030 - C/JAVA ASYNCHRONOUS XPLUGIN TESTS CAUSES CRASH IN MYSQL SERVER 8.0.12

Follow-up fix for build with -DWITH_ICU=system.

Author: lkotula

plugin/x/src/xpl_regex.cc

@@ -53,7 +53,7 @@ bool xpl::Regex::match(const char *value) const {
    * and parse the text patter each time that xpl::Regex::match
    * is called.
    */
-  UnicodeString value_as_utf8{icu::UnicodeString::fromUTF8(value)};
+  icu::UnicodeString value_as_utf8{icu::UnicodeString::fromUTF8(value)};
   std::unique_ptr<icu::RegexMatcher> regexp{
       m_pattern->matcher(value_as_utf8, match_status)};