drivers/gna: No empty structs

Empty structs are incompatible when building with C++, which specifies
their size as 1 byte.  Technically they are illegal in C; GCC has
always supported them as an extension, but with a size of *zero*
bytes.

The upshot is that we can't have them in headers that may be presented
to a C++ compiler.  Add a placeholder field.

We only get the resulting compiler warning with -std=c++98 it seems
like, which is why this went undetected.

Signed-off-by: Andy Ross <andyross@google.com>