Fix recently broken wxRegEx build in UTF-8 mode

Restore the use of c_str() replaced with wx_str() in 4dd77dabe8 (Check
for WXREGEX_CONVERT_TO_MB when calling regcomp() too, 2021-07-16), as
it's still necessary when wxUSE_UNICODE_UTF8==1.

src/common/regex.cpp

@@ -550,7 +550,7 @@ bool wxRegExImpl::Compile(const wxString& expr, int flags)
         flagsRE |= REG_NEWLINE;
 
 #ifndef WXREGEX_CONVERT_TO_MB
-    const wxChar *exprstr = expr.wx_str();
+    const wxChar *exprstr = expr.c_str();
 #else
     const wxScopedCharBuffer exprbuf = expr.utf8_str();
     const char* const exprstr = exprbuf.data();