core/utils: fix installDirectory method in linux

To find the directory where is located the binary, sibr::Utils::getInstallDirectory method uses readlink() call.

In some cases, the returned executable path includes additional characters that makes the binary execution fails.

readlink() does not null terminate the string, so we make sure that the string terminates at the length returned by readlink.

Signed-off-by: Cedric CHEDALEUX cedric.chedaleux@orange.com