Make 'size_t' a class 3 keyword in C

This is consistent with how we highlight recognized identifiers such as 'Real' in Modelica listings.

mlsshared.sty

@@ -135,7 +135,8 @@
         else,enum,extern,float,for,goto,if,int,long,register,return,%
         short,signed,sizeof,static,struct,switch,typedef,union,unsigned,%
         void,volatile,while},
-    % henrikt-ma: How about adding some highlighting of 'size_t' as a recognized name?
+    morekeywords=[3]{%
+        size_t},
     sensitive=true,
     morecomment=[s]{/*}{*/},
     morecomment=[l]//, % nonstandard