Fix build on powerpc64(le)

[pkubaj]

dir should be signed, same as a class variable dirB. Otherwise build fails on PPC:

src/tigr/postnuc.cc:174:37: error: non-constant-expression cannot be narrowed from type 'signed char' to 'char' in initializer list [-Wc++11-narrowing]
        Alignments.push_back({ *Mp, CurrCp->dirB } );
                                    ^~~~~~~~~~~~
src/tigr/postnuc.cc:174:37: note: insert an explicit cast to silence this issue
        Alignments.push_back({ *Mp, CurrCp->dirB } );
                                    ^~~~~~~~~~~~
                                    static_cast<char>( )
1 error generated.