selftests/Makefile: include $(INSTALL_DEP_TARGETS) in clean target to clean net/lib...

selftests/Makefile: include $(INSTALL_DEP_TARGETS) in clean target to clean net/lib dependency

The selftests 'make clean' does not clean the net/lib because it only
processes $(TARGETS) and ignores $(INSTALL_DEP_TARGETS). This leaves
compiled objects in net/lib after cleaning, requiring manual cleanup.

Include $(INSTALL_DEP_TARGETS) in clean target to ensure net/lib
dependency is properly cleaned.

Signed-off-by: Nai-Chen Cheng <bleach1827@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Simon Horman <horms@kernel.org> # build-tested
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://patch.msgid.link/20250910-selftests-makefile-clean-v1-1-29e7f496cd87@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>