add minimal player.h (fixes compiler errors)

[RossBencina]

The following files #include player.h:

example10-5.c, example10-6.c, example10-7.c, example10-8.c

but no player.h file is present in the repository. In-fact, the contents of player.h are contained in example10-4.c.

This fix adds a minimal player.h that #includes example10-4.c so that '.c' files that #include player.h will compile.