Add 'const' qualifier to mapset string (#1276)

Fixes -Wincompatible-pointer-types-discards-qualifiers
compiler warnings.

raster/r.fill.stats/main.c

@@ -743,7 +743,7 @@ int main(int argc, char *argv[])
     /* program settings */
     char *input;
     char *output;
-    char *mapset;
+    const char *mapset;
     double radius = 1.0;
     unsigned long min_cells = 12;
     double power = 2.0;

raster3d/r3.retile/main.c

@@ -87,7 +87,7 @@ int main(int argc, char *argv[])
     struct GModule *module;
     RASTER3D_Map *map = NULL;
     int tileX, tileY, tileZ;
-    char *mapset;
+    const char *mapset;
 
     /* Initialize GRASS */
     G_gisinit(argv[0]);