fix initializer warning with arma::vec

apparently different versions of gcc were raising warnings with the
assignment of values in neighbors. The recommended fix was to use an
initializer list, which is explained in the arma doc below.

https://arma.sourceforge.net/docs.html#element_initialisation

Author: ahgamut

src/ThinImageCpp.cpp

@@ -203,7 +203,7 @@ void count246(const arma::mat &img, uvec checkList, vec &toWhite)
   for(int i = 0; i < checkList.n_elem; i++)
   {
     cell = checkList[i];
-    neighbors << img[cell - 1] << img[cell + n] << img[cell + 1];
+    neighbors = { img[cell - 1], img[cell + n], img[cell + 1] };
     if(any(neighbors == 1))
       toWhite[i] = 1;
   }