The package links against libusb, so this should be reflected in its dependencies.
To be precise, it links against libusb if libusb is present at build time.
PS: Alas there does not seem to be any ./configure switch to disable building with libusb when libusb is present. (libusb is only required for some cameras of one vendor which implement a variant of the IEEE 1394 IIDC protocol alias DCAM over the USB interface.)
+ 13 Sep 2011; Samuli Suominen <ssuominen@gentoo.org> + libdc1394-2.1.3-r1.ebuild, metadata.xml: + Missing virtual/libusb:1 and dev-util/pkgconfig deps wrt #341601 by Christian + Schmitt.