package/sdl_sound: actually use the optional CONF_OPTS

Since "57ace26b package/sdl_sound: add optional support for
libmodplug" from 2016, optional CONF_OPTS are added but they do not
really take effect because there is an unconditional override below the
conditional append.

Currently this does not cause build failures, but it can lead to wrong
detection of dependencies because many explicit --enable/--disable are
not passed to configure.

Fix this by moving the unconditional code to the top.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>