Always only add to EXTRA_DIST, not set it anew
Always only add to EXTRA_DIST, not set it anew

As EXTRA_DIST is already defined in Makefile.am, we can only append
to (not assign to) EXTRA_DIST in e.g. usb/Makefile-files. This fixes
the !HAVE_UDEV case to use += instead of =.


git-svn-id: 40dd595c6684d839db675001a64203a1457e7319@12576 67ed7778-7388-44ab-90cf-0a291f65f57c

gphoto/libgphoto2.OLDMIGRATION,gphoto/libgphoto2.OLDMIGRATION,gphoto/libgphoto2.OLDMIGRATION,gphoto/libgphoto2.OLDMIGRATION