i2c-pseudo: build for kernels >5.15
i2c-pseudo: build for kernels >5.15

Linux 5.15 moved stdarg.h to linux/stdarg.h, and removed the old
stdarg.h in commit c0891ac15f0428ffa81b2e818d416bdf3cb74ab6. Update
the i2c-pseudo module to build on these newer kernels.

BUG=none
TEST=now builds on Linux 5.16.18
BRANCH=none

Change-Id: I331cfc3c2bb7300b5f2c19d595214bfb1e9a8622
Signed-off-by: Peter Marheine <ef0b2c492edde94778d7465bfbaeed52c5be6858@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3650292
Reviewed-by: Shou-Chieh Hsu <f5ae2e797db2f48ecd5b3f6c55380c4e2acbd783@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec