avoid redefining DECLSPEC_DEPRECATED

fixes, e.g.:

In file included from ../vpx/internal/../vpx_decoder.h:33:0,
                 from ../vpx/internal/vpx_codec_internal.h:46,
                 from ../vp8/common/onyx.h:21,
                 from ../vp8/encoder/block.h:15,
                 from ../test/subtract_test.cc:18:
../vpx/internal/../vpx_codec.h:52:0: warning: "DECLSPEC_DEPRECATED"
redefined
/usr/x86_64-w64-mingw32/sys-root/mingw/include/winnt.h:164:0: note: this
is the location of the previous definition

Change-Id: Iddc9318451d3e4e4a78b4d706518083fffff5c61