build: rename version.h to libavutil/ffversion.h

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>

.gitignore

@@ -27,7 +27,6 @@
 /ffserver
 /config.*
 /coverage.info
-/version.h
 /doc/*.1
 /doc/*.3
 /doc/*.html
@@ -53,6 +52,7 @@
 /libavcodec/*_tables.c
 /libavcodec/*_tables.h
 /libavutil/avconfig.h
+/libavutil/ffversion.h
 /tests/audiogen
 /tests/base64
 /tests/data/