Fix pkg-config for static linking

[joerg-krause]

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.

[mpruett]

Thanks, Jörg. I don't believe there's a need for including coverage in production builds, so I've modified your change to add only FLAC to Libs.private in audiofile.pc.in. The change has been applied in 82f9131.