Fix build with gcc6

gcc6 triggers an error during build:

xsid.cpp:101:1: error: narrowing conversion of ''\200'' from 'char' to 'int8_t {aka signed char}' inside { } [-Wnarrowing]
 };
 ^
xsid.cpp:101:1: error: narrowing conversion of ''\224'' from 'char' to 'int8_t {aka signed char}' inside { } [-Wnarrowing]
xsid.cpp:101:1: error: narrowing conversion of ''\251'' from 'char' to 'int8_t {aka signed char}' inside { } [-Wnarrowing]
xsid.cpp:101:1: error: narrowing conversion of ''\274'' from 'char' to 'int8_t {aka signed char}' inside { } [-Wnarrowing]
xsid.cpp:101:1: error: narrowing conversion of ''\316'' from 'char' to 'int8_t {aka signed char}' inside { } [-Wnarrowing]
xsid.cpp:101:1: error: narrowing conversion of ''\341'' from 'char' to 'int8_t {aka signed char}' inside { } [-Wnarrowing]
xsid.cpp:101:1: error: narrowing conversion of ''\362'' from 'char' to 'int8_t {aka signed char}' inside { } [-Wnarrowing]

To fix the problem change the array type from int8_t to char as proposed
by Khem Raj:
http://lists.busybox.net/pipermail/buildroot/2016-August/169540.html

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>

diff -uNr sidplay-libs-2.1.1.org/libsidplay/src/xsid/xsid.cpp sidplay-libs-2.1.1/libsidplay/src/xsid/xsid.cpp
--- sidplay-libs-2.1.1.org/libsidplay/src/xsid/xsid.cpp	2004-06-14 22:08:02.000000000 +0200
+++ sidplay-libs-2.1.1/libsidplay/src/xsid/xsid.cpp	2016-08-12 18:10:20.000000000 +0200
@@ -94,7 +94,7 @@
     '\x08', '\x19', '\x2a', '\x3b', '\x4c', '\x5d', '\x6e', '\x7f'
 };
 */
-const int8_t XSID::sampleConvertTable[16] =
+const char XSID::sampleConvertTable[16] =
 {
     '\x80', '\x94', '\xa9', '\xbc', '\xce', '\xe1', '\xf2', '\x03',
     '\x1b', '\x2a', '\x3b', '\x49', '\x58', '\x66', '\x73', '\x7f'
diff -uNr sidplay-libs-2.1.1.org/libsidplay/src/xsid/xsid.h sidplay-libs-2.1.1/libsidplay/src/xsid/xsid.h
--- sidplay-libs-2.1.1.org/libsidplay/src/xsid/xsid.h	2004-06-14 22:08:02.000000000 +0200
+++ sidplay-libs-2.1.1/libsidplay/src/xsid/xsid.h	2016-08-12 18:11:01.000000000 +0200
@@ -238,7 +238,7 @@
     uint8_t             sidData0x18;
     bool                _sidSamples;
     int8_t              sampleOffset;
-    static const int8_t sampleConvertTable[16];
+    static const char sampleConvertTable[16];
     bool                wasRunning;
 
 private: