Fix compiler error due to missing pthread header (Bugfix #2953)

Compiling current master (e27f4da9) on Debian Squeeze results in the following compilation error:

[ 52%] Building CXX object modules/highgui/CMakeFiles/opencv_highgui.dir/src/cap_ffmpeg.cpp.o
In file included from /home/shuber/src/opencv/modules/highgui/src/cap_ffmpeg.cpp:45:
/home/shuber/src/opencv/modules/highgui/src/cap_ffmpeg_impl.hpp: In member function 'void ImplMutex::Impl::init()':
/home/shuber/src/opencv/modules/highgui/src/cap_ffmpeg_impl.hpp:417: error: 'pthread_spin_init' was not declared in this scope
/home/shuber/src/opencv/modules/highgui/src/cap_ffmpeg_impl.hpp: In member function 'void ImplMutex::Impl::destroy()':
/home/shuber/src/opencv/modules/highgui/src/cap_ffmpeg_impl.hpp:418: error: 'pthread_spin_destroy' was not declared in this scope
/home/shuber/src/opencv/modules/highgui/src/cap_ffmpeg_impl.hpp: In member function 'void ImplMutex::Impl::lock()':
/home/shuber/src/opencv/modules/highgui/src/cap_ffmpeg_impl.hpp:420: error: 'pthread_spin_lock' was not declared in this scope
/home/shuber/src/opencv/modules/highgui/src/cap_ffmpeg_impl.hpp: In member function 'bool ImplMutex::Impl::trylock()':
/home/shuber/src/opencv/modules/highgui/src/cap_ffmpeg_impl.hpp:421: error: 'pthread_spin_trylock' was not declared in this scope
/home/shuber/src/opencv/modules/highgui/src/cap_ffmpeg_impl.hpp: In member function 'void ImplMutex::Impl::unlock()':
/home/shuber/src/opencv/modules/highgui/src/cap_ffmpeg_impl.hpp:422: error: 'pthread_spin_unlock' was not declared in this scope
make²: * [modules/highgui/CMakeFiles/opencv_highgui.dir/src/cap_ffmpeg.cpp.o] Error 1
make¹: [modules/highgui/CMakeFiles/opencv_highgui.dir/all] Error 2
make: ** [all] Error 2

The attached patch file fixes this.