mplayer: add license info

Most code is GPLv2+ or compatible except for one file that is
GPLv2-only, hence as a whole is GPLv2 (see Copyright).
Also kill some whitespace and make the only hard dependency
(host-pkgconf) a hard dependency, not an addition, since it's not
conditional.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>