ANDROID: dm-user: Add some missing static

I forgot to add static to a handful of dm-user function declarations.
Not sure how I missed these warnings the other times, but the 0-day
robot found them on android-mainline.

Reported-by: kernel test robot <lkp@intel.com>
Test: none
Fixes: 83bf345abc0b ("ANDROID: dm: dm-user: New target that proxies BIOs to userspace")
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Change-Id: If9cb2de117ff402c86902d0d2d6a2f0e28614c54