italicparser: Parse(): Add static keyword to regex

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

AUTHORS

@@ -9,3 +9,4 @@ Patrick José Pereira (patrickelectric@gmail.com)
 Martin Kopecky (martin.kopecky357@gmail.com)
 Andrew Mettlach (dmmettlach@gmail.com)
 Evan Klitzke (evan@eklitzke.org)
+Albert Schwarzkopf (dev-maddy@quitesimple.org)

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);
   }