new symbol: barb is 37% of the shaft length @ 28 degrees

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@48339 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/symbol/symbol/geology/half_arrow

@@ -0,0 +1,9 @@
+VERSION 1.0
+BOX -1 -1 1 1
+STRING
+  LINE
+    -1 0
+     1 0
+     0.3466 0.3474
+  END
+END