LineBufferOverlay: Result is not floating-point

dec/2 is an integer and calling round() doesn't make sense, dec/2.0 is a double

From coverity 'integer_division: Dividing integer expressions dec and 2,
and then converting the integer quotient to type double.
Any remainder, or fractional part of the quotient, is ignored.'

src/LineBufferOverlay.cpp

@@ -153,9 +153,9 @@ void LineBufferOverlay::setLineBuffer()
 
         arrow.pushLine( 0, dec,  0, dec - windArrowSize );   // hampe
         // Right arrow
-        arrow.pushLine(0, dec - ceil(m_lineWidth / 2 * sqrt(2)), round(dec/4 + (m_lineWidth / 2 * sqrt(2))), round(dec/2));
+        arrow.pushLine(0, dec - ceil(m_lineWidth / 2 * sqrt(2)), round(dec/4 + (m_lineWidth / 2 * sqrt(2))), round(dec/2.));
         // Left arrow
-        arrow.pushLine(0, dec - ceil(m_lineWidth / 2 * sqrt(2)), round(-dec/4 - (m_lineWidth / 2 * sqrt(2))), round(dec/2));
+        arrow.pushLine(0, dec - ceil(m_lineWidth / 2 * sqrt(2)), round(-dec/4 - (m_lineWidth / 2 * sqrt(2))), round(dec/2.));
     }
 
     float alpha = 0.38;