libosmocore: Add libusb to DEPENDS

In I656a1a38cbb5b1f3a9145d2869d3b4d0adefcae3, libosmocore got libusb
integration support, and since then libusb is a dependency (which can be
disabled through --disable-libusb).

Related: OS#4299
Change-Id: I86773a3df7f249fae565bd15d8761a537a79651e
1 file changed