[Makefile] Add "+" to end of version string for git-generated images

This "+" should be removed when a tarball release is done.

src/Makefile.housekeeping

 VERSION_MAJOR	= 0
 VERSION_MINOR	= 9
 VERSION_PATCH	= 3
-EXTRAVERSION	=	
+EXTRAVERSION	= +
 MM_VERSION	= $(VERSION_MAJOR).$(VERSION_MINOR)
 VERSION		= $(MM_VERSION).$(VERSION_PATCH)$(EXTRAVERSION)
 CFLAGS		+= -DVERSION_MAJOR=$(VERSION_MAJOR) \