core-image-rt-sdk: make sure that we append to DEPENDS

This also fix postinst intercept hook 'update_gio_module_cache' failed warnings
that are now flagged as errors after the recent chagnes at lib/oe/package_manager.py

(From OE-Core rev: 6ba5f32333e9ed6be980ca2b71b547f795e83497)

Signed-off-by: Naveen Saini <naveen.kumar.saini@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-rt/images/core-image-rt-sdk.bb

@@ -11,7 +11,7 @@ python () {
 DESCRIPTION = "Small image capable of booting a device with a test suite and \
 tools for real-time use. It includes the full meta-toolchain, development \
 headers and libraries to form a standalone SDK."
-DEPENDS = "linux-yocto-rt"
+DEPENDS += "linux-yocto-rt"
 
 IMAGE_FEATURES += "dev-pkgs tools-sdk tools-debug eclipse-debug tools-profile tools-testapps debug-tweaks"