Export android_pmem.h to keep userspace compiling

Some applications still include android_pmem.h although all the
functionality for it is gone from the kernel. Keep this header
file around until all such users have removed the include.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>