[cmake] Add dependency on llvm-dwarfdump to llvm-locstats

Though llvm-locstat is a "util" it requires llvm-dwarfdump
to be useful. So what happens now is that unless LLVM_BUILD_TOOLS
is ON llvm-locstats won't be part of the "all" target.

However given that it requires llvm-dwarfdump, this is less
confusing/build time consuming than suddenly having llvm-dwarfdump
build when all other tools are disabled.

Differential Revision: https://reviews.llvm.org/D71705