headers: Use a custom macro for declaring extern const variables

This fixes the same warning as d551b7d85ec63c0fa33f1a51ec5157722709fc53
(warnings due to declaring a variable as extern and initializing it
at the same time) tried to fix, without overriding the EXTERN_C macro,
which affected other headers.

Signed-off-by: Martin Storsjö <martin@martin.st>
2 files changed