waffle: define WAFFLE_API for Windows platforms

Allows us to properly annotate the functions and
export only the required ones.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>
core: update wcore_enum_to_string()

Add a couple of missing cases - PLATFORM_GBM and DL_OPENGL_ES3.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>