added missing headers

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@39422 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/vector/Vlib/buffer2.c

@@ -18,6 +18,7 @@
 
 #include <grass/config.h>
 #include <stdlib.h>
+#include <math.h>
 #include <grass/vector.h>
 #include <grass/gis.h>
 #include <grass/glocale.h>

lib/vector/Vlib/dgraph.c

@@ -15,6 +15,8 @@
 
 #include <grass/config.h>
 #include <stdlib.h>
+#include <string.h>
+#include <math.h>
 #include <grass/vector.h>
 #include <grass/gis.h>
 #include <grass/glocale.h>