setting g_xbc7_dc_quant_per_weight_bits to const (very minor fix)

transcoder/basisu_xbc7_decoder.h

diff --git a/transcoder/basisu_xbc7_decoder.h b/transcoder/basisu_xbc7_decoder.h
index 3b9a106..96780fb 100644
--- a/transcoder/basisu_xbc7_decoder.h
+++ b/transcoder/basisu_xbc7_decoder.h
@@ -633,7 +633,7 @@
 	// reconstruction snaps to a 2^n-level grid (step ~64/(2^n - 1) in [0,64]
 	// space), so coarse planes tolerate proportionally coarser DC. Same
 	// build-alike caveat as above.
-	static bool g_xbc7_dc_quant_per_weight_bits = true;
+	static const bool g_xbc7_dc_quant_per_weight_bits = true;
 
 	static inline int get_xbc7_dc_quant(uint32_t num_weight_bits)
 	{