tools: imx8mimage: Fix potential memory leak

Dynamic memory, referenced by 'line', is allocated at imx8mimage.c:187
by calling function 'getline' and lost at imx8mimage.c:210.

Signed-off-by: Maks Mishin <maks.mishinFZ@gmail.com>
diff --git a/tools/imx8mimage.c b/tools/imx8mimage.c
index d60d293..0f24ba7 100644
--- a/tools/imx8mimage.c
+++ b/tools/imx8mimage.c
@@ -206,6 +206,7 @@
 		}
 	}
 
+	free(line);
 	fclose(fd);
 	return 0;
 }