base: Define PERFETTO_COMPONENT_EXPORT conditionally
base: Define PERFETTO_COMPONENT_EXPORT conditionally

Only define the PERFETTO_COMPONENT_EXPORT macro if the embedder hasn't
already done so. This allows the embedder to customize Perfetto symbol
visibility in SDK builds.

Bug: b:234546493
Test: manual
Change-Id: Ib495fbab493960ed01044838f89d994eaaf8a04c
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto