[m-dev.] trivial diff: mercury_trace_external.c portability fix

[Erwan Jahier]

Fergus Henderson wrote:
> 
> runtime/mercury_trace_external.c:
>         Move the #include of nonportable header files such as
>         <arpa/inet.h>, etc. inside the `#ifdef MR_USE_EXTERNAL_DEBUGGER'.
> 


In what sense do you say they are not portable ?

The fact is I needed to comment out #include <arpa/inet.h> and #include
<netinet/in.h> to make things compiling here (delaying that problem the
day I want to support inet communication...) because gcc was complaining
about code in arpa/inet.h and netinet/in.h

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R1.