Fix duplicate function prototype

It seems that the duplicate prototype snuck in during the 8.4 merge,
which should've been mostly a noop as I had synced pgcrypto with 8.4
in 16d26ee9. I had however placed the prototype on the wrong line
and so git merge was able to cleanly place the protype a few lines
down, creating a duplicate.

contrib/pgcrypto/px.h

@@ -199,7 +199,6 @@ const char *px_strerror(int err);
 const char *px_resolve_alias(const PX_Alias *aliases, const char *name);
 
 void		px_set_debug_handler(void (*handler) (const char *));
-void		px_memset(void *ptr, int c, size_t len);
 
 void		px_memset(void *ptr, int c, size_t len);