clang compile error because of unused variable

There's an unused variable appeared in ui/clipboard.c:21. It will cause an compiling failure when CC=clang CXX=clang++ make due to unused variable. This variable is introduced in this commit .

There're also 2 ununsed variables in target/hexagon/mmvec/macros.h, which introduced by log_bank in GATHER_FUNCTION macro and log_bank in SCATTER_FUNCTION macro.

After remove these variables, the compiling process worked. I still don't know whether they have any side effect. Although clang may not be officially supported by qemu, we still need to remove them? I would like to help this when needed.