Another FFmpeg fix from master.

From commit dd74a851, to be exact. Now cap_ffmpeg.cpp should actually
build if HAVE_FFMPEG is true.

Also modified some gpu sources in a similar manner.

modules/gpu/src/ffmpeg_video_source.cpp

@@ -44,7 +44,7 @@
 
 #if defined(HAVE_CUDA) && defined(HAVE_NVCUVID)
 
-#if defined(HAVE_FFMPEG) && defined(BUILD_SHARED_LIBS)
+#if defined(HAVE_FFMPEG) && defined(BUILD_SHARED_LIBS) && !defined(WIN32)
     #include "../src/cap_ffmpeg_impl.hpp"
 #else
     #include "../src/cap_ffmpeg_api.hpp"

modules/gpu/src/video_writer.cpp

@@ -70,12 +70,7 @@ void cv::gpu::VideoWriter_GPU::EncoderParams::save(const std::string&) const { t
 
 #else // !defined HAVE_CUDA || !defined WIN32
 
-#ifdef HAVE_FFMPEG
-    #include "../src/cap_ffmpeg_impl.hpp"
-#else
-    #include "../src/cap_ffmpeg_api.hpp"
-#endif
-
+#include "../src/cap_ffmpeg_api.hpp"
 
 ///////////////////////////////////////////////////////////////////////////
 // VideoWriter_GPU::Impl

modules/highgui/src/cap_ffmpeg.cpp

@@ -41,7 +41,7 @@
 
 #include "precomp.hpp"
 
-#ifdef HAVE_FFMPEG
+#if defined HAVE_FFMPEG && !defined WIN32
 #include "cap_ffmpeg_impl.hpp"
 #else
 #include "cap_ffmpeg_api.hpp"