package/mpd: fix static build with tremor and vorbis

package/mpd: fix static build with tremor and vorbis

mpd can't be built statically with tremor and vorbis, build fails on:
block.c:(.text+0x11c): multiple definition of `vorbis_block_init'
/usr/lfs/v0/rc-buildroot-test/scripts/instance-2/output/host/bin/../sparc-buildroot-linux-uclibc/sysroot/usr/lib/libvorbisidec.a(block.o):block.c:(.text+0x0): first defined here
/usr/lfs/v0/rc-buildroot-test/scripts/instance-2/output/host/opt/ext-toolchain/bin/../lib/gcc/sparc-buildroot-linux-uclibc/7.4.0/../../../../sparc-buildroot-linux-uclibc/bin/ld: Disabling relaxation: it will not work with multiple definitions
/usr/lfs/v0/rc-buildroot-test/scripts/instance-2/output/host/bin/../sparc-buildroot-linux-uclibc/sysroot/usr/lib/libvorbis.a(block.o): In function `_vorbis_block_alloc':

Fix this by making both options mutually exclusive

It should be noted that upstream explicitly removed the possibility to
enable tremor and vorbis at the same time since version 0.21.6 and
https://github.com/MusicPlayerDaemon/MPD/commit/c18cd941aaa4c16ac713bc4f1e16ada5281d689e
Since we only have 0.20.23, it is still possible to build them together,
but there is clearly no need to allow it.

Fixes:
 - http://autobuild.buildroot.org/results/60c721a82ffd668bebf02d80bca83780d6cdb2f2

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>