examples: Don't require msgfmt when compiling hello-c-gnome3
* hello-c-gnome3/Makefile.am (CLEANFILES): Clean hello.desktop.
(EXTRA_DIST): Add hello.desktop.in.
(MAINTAINERCLEANFILES): Add hello.desktop.in.
(hello.desktop): Move msgfmt invocation to...
(hello.desktop.in): ...here.
