Fix a -Wundef warning in cl_platform.h

`_MSC_VER` -> `defined _MSC_VER`. Should fix building with MinGW. Similar to fbc91c5ee.

Example of warning: http://build.opencv.org/builders/win_x86%5Brelease%5D--%5Bsse_%3D_ON%5D--%5Btbb_%3D_OFF%5D--%5Bgpu_%3D_OFF%5D--%5Bcompiler_%3D_mingw%5D--%5Btests_%3D_ON%5D-/builds/1227/steps/compile%20release%2032%20mingw%20shared%20n/logs/build%20summary%20%28e%3A%200%2C%20w%3A%202%29