floss: use pkg-config for linking protobuf and flatbuffers

These libraries provide pkg-config files, so use them for the build and
link flags since newer versions of protobuf include linker flags for
abseil-cpp libraries.

Bug: 303101747
Test: mmm packages/modules/Bluetooth
  # and emerge-${BOARD} floss
Change-Id: I247784a98b433f9f999342c680ea1bbe7476a106