Fix big endian build

Fix big endian build

I get the following error when building fio on PowerPC:

client.c: In function ‘convert_agg’:
client.c:641:22: error: invalid operands to binary & (have ‘long double’ and ‘long long unsigned int’)

It looks like we have things backwards, we should byteswap the
integer value before we convert it to a double.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>