Build SF executable with USE_HWC2 when needed

The SurfaceFlinger executable wasn't picking up the USE_HWC2 flag when
enabled by the TARGET_USES_HWC2 BoardConfig, causing a mismatch in the
size of the SurfaceFlinger object. This fixes that by also passing
USE_HWC2 when building the executable.

Test: Cherry-pick from internal branch
Bug: 30302693
Change-Id: I1e41e5f548d4b8329590a082d9b69155ba365be9