package/sdl_mixer: fix static build with tremor

package/sdl_mixer: fix static build with tremor

Static build with tremor raises a build failure since bump to latest git
tree in commit c8e27f3baa90351a417ff6e172d214c4a72e7314. However, it
should be noted that before this bump, tremor was always disabled in
static builds because vorbisidec detection was broken until
https://github.com/libsdl-org/SDL_mixer/commit/565a9a27cc8c184ad0203f004b834880ffd45d32

Fixes:
 - http://autobuild.buildroot.org/results/9634adc433da0e25732eb98675c59d0f96ac93b2

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>