wrt350nv2-builder: Fix memory leak

Add missing call to `free` for variable `buffer` in function
`create_bin_file`.

Signed-off-by: Andrea Dalla Costa <andrea@dallacosta.me>

tools/wrt350nv2-builder/src/wrt350nv2-builder.c

@@ -556,6 +556,7 @@ int create_bin_file(char *bin_filename) {
 		}
 	}
 
+	free(buffer);
 	return exitcode;
 }