exynos: add C++ support to exynos_drmif header

Add the usual extern "C" when compiling in C++ mode.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>