tools: ynl-gen: don't output external constants

A definition with a "header" property is an "external" definition
for C code, as in it is defined already in another C header file.
Other languages will need the exact value but C codegen should
not recreate it. So don't output those definitions in the uAPI
header.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250203215510.1288728-1-kuba@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>