Add FFMPEG to include path for OpenMW

I'm a bit confused; `mwsound/ffmpeg_decoder.hpp/cpp` requires FFMPEG headers to compile, how did this work in the first place?
(cherry picked from commit d2a4175804)

apps/openmw/CMakeLists.txt

@@ -108,7 +108,10 @@ endif ()
 
 # Sound stuff - here so CMake doesn't stupidly recompile EVERYTHING
 # when we change the backend.
-include_directories(${SOUND_INPUT_INCLUDES})
+include_directories(
+    ${SOUND_INPUT_INCLUDES}
+    ${FFMPEG_INCLUDE_DIRS}
+)
 
 target_link_libraries(openmw
     ${OENGINE_LIBRARY}