compiler: Suppress sparse complaints about function attributes.

compiler: Suppress sparse complaints about function attributes.

GCC allows __attribute__s to be included in function prototypes and
then omitted later on the function definition, but sparse complains about
this.  Furthermore, sparse doesn't like the placement of the __attribute__s
that we tend to use in OVS.

I don't see any value in "fixing" these to suit sparse so it seems better
to just omit them.