LC3plus high-res flag not added correctly to ISAR bitstream header for External Renderer

Crashes were observed in !2351 after enabling tests for LC3plus high-res mode.

It was discovered that this was due to a forgotten update to the External renderer API function IVAS_REND_GetSplitRendBitstreamHeader() which does not set the flag correctly. The issue does not occur for IVAS_DEC_GetSplitRendBitstreamHeader() since this function correctly sets the value.