Fix parameter mismatch between header and implementation

RIL_onUnsolicitedResponse's second parameter is
const void *data
according to include/telephony/ril.h and rild/rild.c, but
void *data
according to libril/ril.cpp

This mismatch causes a compile failure with clang (and I'm surprised
it compiles with gcc with the mismatch - probably gcc parses this as
function overloading even though it's extern "C", which wouldn't do
what is expected).

Change-Id: Ib437040aeca4be43cc2d327c3715ffa98c76878c
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>