FileSys/Kernel: Implement SetSize service call for File objects.

src/core/file_sys/file.h

@@ -43,6 +43,13 @@ public:
      */
     virtual size_t GetSize() const = 0;
 
+    /**
+     * Set the size of the file in bytes
+     * @param size New size of the file
+     * @return true if successful
+     */
+    virtual bool SetSize(const u64 size) const = 0;
+
     /**
      * Close the file
      * @return true if the file closed correctly