#6 libdkim-1.0.21 fails to compile with GCC-6

[Hodorgasm]

Building libdkim-1.0.21 with gcc-6.3.0 fails with:

dkimverify.cpp:221:98: error: narrowing conversion of ‘-1’ from ‘int’ to ‘unsigned char’ inside { } [-Wnarrowing]
 1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1};

See Gentoo Bug 594320

In GCC-6, narrowing conversions are not allowed in braced initialization. Explicit casting must be used.