Fixes the following warnings;
```
cc -Wall -O2 -fPIC -Wall -Wshadow -g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat -Wformat-security -I/usr/include/iverilog -c -o ipc.o ipc.c
ipc.c: In function ‘ipc_receive’:
ipc.c:98:17: warning: declaration of ‘index’ shadows a global declaration [-Wshadow]
ipc.c:113:17: warning: declaration of ‘index’ shadows a global declaration [-Wshadow]
```
Fixes https://github.com/m-labs/migen/issues/14
From `man iverilog-vpi`;
> iverilog-vpi is a tool to simplify the compilation of VPI modules for use
> with Icarus Verilog. It takes on the command line a list of C or C++ source
> files, and generates as output a linked VPI module.
Fixes https://github.com/m-labs/migen/issues/11
Florent Kermarrec
William D. Jones
Tim 'mithro' Ansell
Sebastien Bourdeauducq
Sebastien Bourdeauducq