selftests: print installation complete message

Add installation complete message to Makefile install logic.

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/Makefile

···
 	else \
 		printf "Unable to get version from git describe\n"; \
 	fi
+	@echo "**Kselftest Installation is complete: $(INSTALL_PATH)**"
 else
 	$(error Error: set INSTALL_PATH to use install)
 endif