package/audiofile: fix compilation with gcc6

package/audiofile: fix compilation with gcc6

Fixes

make[3]: Entering directory '/home/buildroot/br7_freeswitch/output/build/audiofile-0.3.6/libaudiofile/modules'
/bin/bash ../../libtool  --tag=CXX   --mode=compile /home/buildroot/br7_freeswitch/output/host/usr/bin/i586-buildroot-linux-uclibc-g++ -DHAVE_CONFIG_H -I. -I../.. -I./..  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -DNDEBUG -Wall -Wno-multichar    -fvisibility=hidden -fno-rtti -fno-exceptions -fvisibility-inlines-hidden -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os  -c -o ALAC.lo ALAC.cpp
libtool: compile:  /home/buildroot/br7_freeswitch/output/host/usr/bin/i586-buildroot-linux-uclibc-g++ -DHAVE_CONFIG_H -I. -I../.. -I./.. -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -DNDEBUG -Wall -Wno-multichar -fvisibility=hidden -fno-rtti -fno-exceptions -fvisibility-inlines-hidden -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -c ALAC.cpp  -fPIC -DPIC -o .libs/ALAC.o
In file included from ALAC.cpp:29:0:
SimpleModule.h: In instantiation of 'const int signConverter<(FormatCode)0>::kMinSignedValue':
SimpleModule.h:130:52:   required from 'signConverter<Format>::UnsignedType signConverter<Format>::signedToUnsigned::operator()(signConverter<Format>::SignedType) [with FormatCode Format = (FormatCode)0; signConverter<Format>::UnsignedType = unsigned char; signConverter<Format>::SignedType = signed char]'
/home/buildroot/br7_freeswitch/output/host/usr/i586-buildroot-linux-uclibc/include/c++/6.1.0/bits/stl_algo.h:4184:24:   required from '_OIter std::transform(_IIter, _IIter, _OIter, _UnaryOperation) [with _IIter = const signed char*; _OIter = unsigned char*; _UnaryOperation = signConverter<(FormatCode)0>::signedToUnsigned]'
SimpleModule.h:104:16:   required from 'void transform(const void*, void*, size_t) [with UnaryFunction = signConverter<(FormatCode)0>::signedToUnsigned; size_t = unsigned int]'
SimpleModule.h:176:62:   required from 'static void ConvertSign::convertSignedToUnsigned(const void*, void*, size_t) [with FormatCode Format = (FormatCode)0; size_t = unsigned int]'
SimpleModule.h:183:51:   required from here
SimpleModule.h:126:40: error: left operand of shift expression '(-1 << 7)' is negative [-fpermissive]
  static const int kMinSignedValue = -1 << kScaleBits;
                                     ~~~^~~~~~~~~~~~~

To reproduce the build error use this defconfig:

BR2_GCC_VERSION_6_X=y
BR2_TOOLCHAIN_BUILDROOT_CXX=y
BR2_PACKAGE_AUDIOFILE=y

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>