package/audiofile: add patch to fix static linking issue

Static linking userspace programs such as MPD against libaudiofile fails if
FLAC is available, because libaudiofile is linked against FLAC, but this isn't
expressed in the pkg-config file:

[..]
arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libaudiofile.a(FLAC.o): In function `FLACDecoder::reset2()':
FLAC.cpp:(.text+0x58): undefined reference to `FLAC__stream_decoder_seek_absolute'
/home/buildroot/build/instance-1/output/host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libaudiofile.a(FLAC.o): In function `FLACEncoder::sync2()':
FLAC.cpp:(.text+0x88): undefined reference to `FLAC__stream_encoder_finish'
/home/buildroot/build/instance-1/output/host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libaudiofile.a(FLAC.o): In function `FLACDecoder::~FLACDecoder()':
FLAC.cpp:(.text+0xc4): undefined reference to `FLAC__stream_decoder_delete'
/home/buildroot/build/instance-1/output/host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libaudiofile.a(FLAC.o): In function `FLACEncoder::~FLACEncoder()':
FLAC.cpp:(.text+0x164): undefined reference to `FLAC__stream_encoder_delete'
/home/buildroot/build/instance-1/output/host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libaudiofile.a(FLAC.o): In function `FLACDecoder::runPull()':
[..]

The Libs.private field is specifically designed for such usage:

>From pkg-config documentation:

  Libs.private:

    This line should list any private libraries in use.  Private
    libraries are libraries which are not exposed through your
    library, but are needed in the case of static linking.

Therefore, this patch adds a reference to FLAC as well as to lcov in the
Libs.private field of the pkg-config file.

Patch status: Pending
https://github.com/mpruett/audiofile/pull/26

Fixes:
http://autobuild.buildroot.net/results/e95/e959e703a8db81032da27bece295c121e53d830d/
http://autobuild.buildroot.net/results/10b/10bd00209f098e8782016355cb2f46d1d3e7d3a3/



Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>