build: Prepare config.h for inclusion in examples/tests

build: Prepare config.h for inclusion in examples/tests

There are certain games played in the examples and tests source to
account for differences in build environments and target platforms. This
can be simplified by including config.h and using the definitions there.

To that end, move the printf function attribute definition from
libusbi.h to config.h and leverage it where it is used in the examples
and tests.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>