build: ignore/clean usr/

This directory is generated by the install-crit target, so let's .gitignore
it and clean it on `make clean`.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

.gitignore

@@ -33,3 +33,4 @@ protobuf-desc-gen.h
 criu.pc
 build
 .gitid
+usr/

Makefile

@@ -284,6 +284,7 @@ clean: clean-built
 	$(Q) $(MAKE) -C pycriu $@
 	$(Q) $(RM) ./*.pyc
 	$(Q) $(RM) -r build
+	$(Q) $(RM) -r usr
 
 distclean: clean
 	$(E) "  DISTCLEAN"