[shotwell] interfaces.mk: fix failure to remove non-existing files

[Jens Georg <jensgeorg src gnome org>]

commit 7ff3258be56fe9e29df2528f4905ac1c6899a6ab
Author: Andreas Brauchli <a brauchli elementarea net>
Date:   Sat Jun 4 11:09:08 2016 +0200

    interfaces.mk: fix failure to remove non-existing files

 src/plugins/mk/interfaces.mk |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/plugins/mk/interfaces.mk b/src/plugins/mk/interfaces.mk
index 34be1eb..2c8acf8 100644
--- a/src/plugins/mk/interfaces.mk
+++ b/src/plugins/mk/interfaces.mk
@@ -25,5 +25,5 @@ $(PLUGIN_HEADER): $(PLUGIN_VAPI)
 $(PLUGIN_VAPI): $(PLUGIN_INTERFACES) src/plugins/mk/interfaces.mk
        $(call check_valac_version)
        $(VALAC) -c $(VALAFLAGS) -X -DGETTEXT_PACKAGE='"shotwell"' -X -I. $(foreach 
pkg,$(PLUGIN_PKG_REQS),--pkg=$(pkg)) --includedir=plugins --vapi=$@ --header=$(basename $@).h 
$(PLUGIN_INTERFACES)
-       $(foreach src,$(PLUGIN_INTERFACES),`rm $(notdir $(src)).o`)
+       $(foreach src,$(PLUGIN_INTERFACES),`rm -f $(notdir $(src)).o`)