Fix MIPS DSPR2 build failure.

Fixing the failure:
 'TransposeWx8_Fast_MIPS_DSPR2' was not declared in this scope

BUG=none
R=fbarchard@chromium.org

Review URL: https://codereview.chromium.org/1527243002 .

README.chromium

 Name: libyuv
 URL: http://code.google.com/p/libyuv/
-Version: 1555
+Version: 1556
 License: BSD
 License File: LICENSE
 

include/libyuv/rotate_row.h

@@ -68,6 +68,8 @@ void TransposeWx8_Fast_SSSE3(const uint8* src, int src_stride,
                              uint8* dst, int dst_stride, int width);
 void TransposeWx8_MIPS_DSPR2(const uint8* src, int src_stride,
                              uint8* dst, int dst_stride, int width);
+void TransposeWx8_Fast_MIPS_DSPR2(const uint8* src, int src_stride,
+                                  uint8* dst, int dst_stride, int width);
 
 void TransposeWx8_Any_NEON(const uint8* src, int src_stride,
                            uint8* dst, int dst_stride, int width);