Remove arg from memset macro

My clang doesn't like the macro defined this way, choking at the callsite on line 195 with "too many arguments provided to function-like macro invocation"

This change matches what is done for STBTT_memset in stb_truetype.h

stb_dxt.h

@@ -88,7 +88,7 @@ STBDDEF void stb_compress_bc5_block(unsigned char *dest, const unsigned char *sr
 
 #ifndef STBD_MEMSET
 #include <string.h>
-#define STBD_MEMSET(x)        memset(x)
+#define STBD_MEMSET           memset
 #endif
 
 static unsigned char stb__Expand5[32];