Fixing shadowing of global index function.

Fixing shadowing of global index function.

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