Including libavutil/internal.h breaks compilation of doc/print_options.c
with MSVC due to linking avpriv_strtod/avpriv_snprintf.

This reverts part of commit 095347ff.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>