Up until now, avutil.h includes common.h which includes mem.h which
includes avutil.h, so that all these headers are in fact equivalent.
Yet mem.h does not need to include avutil.h at all and when it no longer
does, including common.h will no longer include error.h (included by
avutil.h) as well; change this by moving error.h to avutil.h, as error.h
is clearly a commonly used header.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>