package/sox: fix static build with libsndfile

package/sox: fix static build with libsndfile

Fix the following static build failure with libsndfile raised since
commit dbc6e9e9f385995eb10bbfb0bb08e7674100ce66:

configure:14720: checking for sf_open_virtual in -lsndfile
configure:14745: /tmp/instance-1/output-1/host/bin/xtensa-buildroot-linux-uclibc-gcc -o conftest -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -mlongcalls -mauto-litpools -Os -g0  -static -Wall -Wmissing-prototypes -Wstrict-prototypes  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -static -Wl,--as-needed conftest.c -lsndfile    >&5
conftest.c:73:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
   73 | char sf_open_virtual ();
      | ^~~~
conftest.c:75:1: warning: function declaration isn't a prototype [-Wstrict-prototypes]
   75 | main ()
      | ^~~~
/tmp/instance-1/output-1/host/lib/gcc/xtensa-buildroot-linux-uclibc/10.3.0/../../../../xtensa-buildroot-linux-uclibc/bin/ld: /tmp/instance-1/output-1/host/xtensa-buildroot-linux-uclibc/sysroot/usr/lib/libsndfile.a(libsndfile_la-flac.o): in function `flac_byterate':
flac.c:(.text+0xfc): undefined reference to `FLAC__StreamDecoderErrorStatusString'

Fixes:
 - http://autobuild.buildroot.org/results/4bc58ed68b29642876bb02710d0cd4f31540de86

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