Created by: qingqing01

Fix compiling error with cuDNN v5:

2:21:26]W:	 [Step 1/5] /paddle/paddle/fluid/platform/cudnn_desc.h:186:19: error: ‘cudnnSetConvolutionMathType’ is not a member of ‘paddle::platform::dynload’
[22:21:26]W:	 [Step 1/5]      CUDNN_ENFORCE(platform::dynload::cudnnSetConvolutionMathType(
[22:21:26]W:	 [Step 1/5]                    ^
[22:21:26]W:	 [Step 1/5] /paddle/paddle/fluid/platform/cudnn_helper.h:65:19: note: in definition of macro ‘CUDNN_ENFORCE’
[22:21:26]W:	 [Step 1/5]      auto status = condition;                                         \
[22:21:26]W:	 [Step 1/5]                    ^
[22:21:26]W:	 [Step 1/5] /paddle/paddle/fluid/platform/cudnn_desc.h:187:15: error: ‘CUDNN_DEFAULT_MATH’ was not declared in this scope
[22:21:26]W:	 [Step 1/5]          desc, CUDNN_DEFAULT_MATH));
[22:21:26]W:	 [Step 1/5]                ^
[22:21:26]W:	 [Step 1/5] /paddle/paddle/fluid/platform/cudnn_helper.h:65:19: note: in definition of macro ‘CUDNN_ENFORCE’
[22:21:26]W:	 [Step 1/5]      auto status = condition;                                         \
[22:21:26]W:	 [Step 1/5]                    ^