avdevice/decklink: fix MSVC build issues

Purpose: Made minor changes to get the decklink avdevice code to build
using Visual C++.

Notes: Made changes to configure per Hendrik Leppkes's review of first
and second versions of patch.  Also made slight alterations per Marton
Balint's reviews.

Comments:

-- configure: Added if enabled decklink section and setting
   decklink_indev_extralibs and decklink_outdev_extralibs here for
   both mingw and Windows.  Also eliminated the setting of these
   variables in the mingw section earlier in the file.

-- libavdevice/decklink_common.cpp: Switched the order of the include
   of libavformat/internal.h to workaround build issues with Visual
   C++.  See comment in file for more details.

-- libavdevice/decklink_dec.cpp:
a) Rearranged the include of libavformat/internal.h (for reasons as
   described above).
b) Made slight alteration to an argument for call to av_rescale_q() to
   workaround a compiler error with Visual C++.  This appears to only
   be an issue when building C++ files with Visual C++.  See comment
   in code for more details.

-- libavdevice/decklink_enc.cpp: Rearranged the include of
   libavformat/internal.h (for reasons as described above).

Signed-off-by: Aaron Levinson <alevinsn@aracnet.com>
Signed-off-by: Marton Balint <cus@passwd.hu>