italicparser: Parse(): Add static keyword to regex

In line with the other parsers, add the static keyword to the
regex.

include/maddy/italicparser.h

@@ -39,7 +39,7 @@ public:
   void
   Parse(std::string& line) override
   {
-    std::regex re("(?!.*`.*|.*<code>.*)\\*(?!.*`.*|.*<\\/code>.*)([^\\*]*)\\*(?!.*`.*|.*<\\/code>.*)");
+    static std::regex re("(?!.*`.*|.*<code>.*)\\*(?!.*`.*|.*<\\/code>.*)([^\\*]*)\\*(?!.*`.*|.*<\\/code>.*)");
     static std::string replacement = "<i>$1</i>";
     line = std::regex_replace(line, re, replacement);
   }