[PATCH]: silence MSVC warning

[Angus Leeming]

The MSVC compiler is complaining about math_rootinset.C:

\lyx\lyx-devel\src\mathed\math_rootinset.C(86) : warning C4805: '==' : 
unsafe mix of type 'DocIterator::idx_type' and type 'bool' in operation

It seems to me that the fix is both obvious and makes the code more 
transparent. Ok to commit?

 bool MathRootInset::idxUpDown(LCursor & cur, bool up) const
 {
-       bool target = !up; // up ? 0 : 1;
+       LCursor::idx_type const target = up ? 0 : 1;
        if (cur.idx() == target)
                return false;
        cur.idx() = target;
        cur.pos() = up ? cur.lastpos() : 0;
        return true;
 }

-- 
Angus