internal/platform_name: move to const char*

Instead of plain char* to avoid need to malloc and do all the needed
error checking needed that goes along with that.

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>

src/mraa.c

@@ -305,7 +305,7 @@ mraa_get_platform_name()
 {
     if (plat == NULL)
         return "Unknown";
-    return plat->platform_name;
+    return (char*) plat->platform_name;
 }
 
 unsigned int