Change all functions referenced in the mxf_metadata_read_table to use the same
prototype and also always call them with exactly those arguments.
The previous way seems to have worked on all supported platforms, however
it was not strictly valid C and would crash e.g. with a compiler that
uses stdcall by default.
Also avoids warnings if -Wstrict-prototypes is used.

Originally committed as revision 23939 to svn://svn.ffmpeg.org/ffmpeg/trunk
