[third_party][sbc] Fix build

Fixes to make the library build:
 - bt_target.h is unnecessary for our use (and has no effect)
 - linker complains about the dequant variables being double-declared
   since they aren't marked extern here.

Test: build with new BUILD.gn rules in fxrev.dev/831276
Change-Id: Ibbf6f5ebfee1a33b8416d00af05a8164329162da