libusb: Add SourceForge mirror.

SourceForge is still getting updated so might as well have it here.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

package/libs/libusb/Makefile

@@ -12,7 +12,9 @@ PKG_VERSION:=1.0.22
 PKG_RELEASE:=1
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.bz2
-PKG_SOURCE_URL:=https://github.com/libusb/libusb/releases/download/v$(PKG_VERSION)
+PKG_SOURCE_URL:=\
+  https://github.com/libusb/libusb/releases/download/v$(PKG_VERSION) \
+  @SF/$(PKG_NAME)
 PKG_HASH:=75aeb9d59a4fdb800d329a545c2e6799f732362193b465ea198f2aa275518157
 
 PKG_INSTALL:=1