init.h: restore designated initializers in SYS_INIT_NAMED()

As seen in the PR #68125 discussion, commit 19a33c78 ("init: adjust
the SYS_INIT dev field init to play nice with older compilers") entirely
threw away designated initializers in SYS_INIT_NAMED() to avoid
compatibility issues across toolchains.

One key aspect that was probably missed at the time: C and C++ are two
different languages and this is especially true with respect to
designated initializers.

Designated initializers provide safer and more readable code, especially
in their much stricter C++ version. So use an #ifdef to restore them in
SYS_INIT_NAMED() thanks to a small braces difference between C and C++.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>