Add check for packed structs in BOOST_COMPUTE_ADAPT_STRUCT()

This adds a compile-time check for non-padded structs in the
BOOST_COMPUTE_ADAPT_STRUCT() macro. Also updates the macro to
add "__attribute__((packed))" to the OpenCL struct definition.
Merge pull request #168 from kylelutz/struct-is-packed

Add check for packed structs in BOOST_COMPUTE_ADAPT_STRUCT()