Also the libavutil/ffversion.h will be installed.

Rationale:
* Applications might want to know FFmpeg's version besides the individual
  libraries'.
* Avoids file name clash between FFmpeg's ./version.h and lib*/version.h when
  a library source file includes both and is compiled on an out-of-tree build.

Fixes #1769.
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>