image_browsers: Add stress-ng to image install

Added the stress-ng package to the IMAGE_INSTALL list to enhance
system stress testing capabilities.

Issue: #38
Change-Type: minor

Author: psaavedra

classes/image_browsers.bbclass

@@ -52,6 +52,7 @@ IMAGE_INSTALL:append = " \
     openssh-sftp \
     openssh-sftp-server \
     smem \
+    stress-ng \
     systemd-analyze \
     valgrind \
     weston-xwayland \