patch 8.1.2120: some MB_ macros are more complicated than necessary

Problem:    Some MB_ macros are more complicated than necessary. (Dominique
            Pelle)
Solution:   Simplify the macros.  Expand inline.
diff --git a/src/highlight.c b/src/highlight.c
index 725affc..0fdd93a 100644
--- a/src/highlight.c
+++ b/src/highlight.c
@@ -4392,7 +4392,7 @@
 		    && col >= shl->startcol
 		    && col < shl->endcol)
 	    {
-		int next_col = col + MB_PTR2LEN(*line + col);
+		int next_col = col + mb_ptr2len(*line + col);
 
 		if (shl->endcol < next_col)
 		    shl->endcol = next_col;