Don't require the huge BC7m6 tables if this format is disabled.

The 1.3 MB file might be considered excessive when bundling the
transcoder sources in another project, don't require it to be present
when not needed.

transcoder/basisu_transcoder.cpp

@@ -146,7 +146,9 @@ namespace basisu
 
 namespace basist
 {
+#if BASISD_SUPPORT_BC7_MODE6_OPAQUE_ONLY
 #include "basisu_transcoder_tables_bc7_m6.inc"
+#endif
 
 #if BASISD_ENABLE_DEBUG_FLAGS
 	static uint32_t g_debug_flags = 0;