gitmodules: Fix tracking branch for GCC 12

A recent commit bumped the GCC version to 12.2,
but adjusting the tracking branch in .gitmodules was forgotten.
Let's fix this and set the tracking branch to gcc-12
(note, that there are no release branches for minor version releases).

Reported-by: Tommy Murphy <tommy_murphy@hotmail.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>