Fix an obsolete file path mentioned in a comment.

src/include/strdup.h

@@ -2,7 +2,7 @@
    with Postgres.  That strdup() is only needed on those systems that
    don't already have strdup() in their system libraries.
 
-   The Postgres strdup() is in src/utils/strdup.c.
+   The Postgres strdup() is in src/port/strdup.c.
 */
 
 extern char *strdup(char const *);