rpc: generalize solution for VPATH builds

rpc: generalize solution for VPATH builds

Commit 5d4b0c4c80 tried to fix certain classes of VPATH builds,
but was too limited.  In particular, Guannan Ren reported:

>    For example: The libvirt source code resides in /home/testuser,
>                 I make dist in /tmp/buildvpath, the XDR routine .c file will
>                 include full path of the header file like:
>
>                 #include "/home/testuser/src/rpc/virnetprotocol.h"
>                 #include "internal.h"
>                 #include <arpa/inet.h>
>
>    If we distribute the tarball to another machine to compile,
>    it will report error as follows:
>
>    rpc/virnetprotocol.c:7:59: fatal error:
>    /home/testuser/src/rpc/virnetprotocol.h: No such file or directory

* src/rpc/genprotocol.pl: Fix more include lines.