configure.ac: find sndfile through pkg-config

configure.ac: find sndfile through pkg-config

Find sndfile through pkg-config to retrieve sndfile dependencies such as
flac and avoid the following static build failure:

/tmp/instance-3/output-1/host/lib/gcc/xtensa-buildroot-linux-uclibc/10.3.0/../../../../xtensa-buildroot-linux-uclibc/bin/ld: /tmp/instance-3/output-1/host/bin/../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/92ed30a6855ca11800b779718822bcba4a69c9a3

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>