openharmony-image-common.inc: Inherit ohos-image

This allows any image including this `.inc` file to have at its disposal
the primitives defined in that class.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

meta-ohos-core/recipes-core/images/openharmony-image-common.inc

 # Common settings for all OpenHarmony images
 
+inherit ohos-image
+
 IMAGE_INSTALL_append = "\
 			packagegroup-net-essentials \
 			packagegroup-net-tools \