powerpc is a platform with 'char' == 'unsigned char'.
As a result '-1' is not expressible in char and build fails as:
```
# '-funsigned-char' to force test build failure on other platforms
$ cmake .. -DBUILD_TESTS=YES -DCMAKE_CXX_FLAGS="-O2 -funsigned-char" -DCMAKE_C_FLAGS="-O2 -funsigned-char"
...
$ make check
tests/test_synchdata.cpp: In member function 'void TestID3v2SynchData::testToUIntBroken()':
tests/test_synchdata.cpp:78:33: error: narrowing conversion of '-1' from 'int' to 'char' inside { } [-Wnarrowing]
char data[] = { 0, 0, 0, -1 };
^
```
The fix is to expliticly cast -1 to 'char'.
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
0b583bafd0