mplayer: enable apng and tdsc only if zlib is available

apng and tdsc are enabled by default, but they need zlib, otherwise the
compilation will fail with an error like this one:

libavcodec/pngdec.c:35:18: fatal error: zlib.h: No such file or
directory
 #include <zlib.h>
                  ^
compilation terminated.

So add an autodep on BR2_PACKAGE_ZLIB to enable or disable support for
apng and tdsc accordingly.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>