vpx_highbd_quantize_b_sse2: fix function proto

Fixes warning with cl.exe:

```
vpx_dsp\x86\highbd_quantize_intrin_sse2.c(25):
  warning C4028: formal parameter 3 different from declaration
```

Change-Id: I699a3c28a2c8481d61be59f00b1085891bf3c975

Author: jzern

vpx_dsp/x86/highbd_quantize_intrin_sse2.c

@@ -19,7 +19,7 @@
 #include "vp9/encoder/vp9_block.h"
 
 void vpx_highbd_quantize_b_sse2(const tran_low_t *coeff_ptr, intptr_t count,
-                                const struct macroblock_plane *mb_plane,
+                                const struct macroblock_plane *const mb_plane,
                                 tran_low_t *qcoeff_ptr, tran_low_t *dqcoeff_ptr,
                                 const int16_t *dequant_ptr, uint16_t *eob_ptr,
                                 const struct ScanOrder *const scan_order) {