modules/hotp-verification: source libusb headers from Makefile DESTDIR

Before, the configure script sourced these from the system FHS
(/usr/include/libusb-1.0). The build failed on my NixOS build machine,
which doesn't store dependencies in a traditional FHS. And this is the
correct approach for reproducible builds.

modules/hotp-verification

@@ -19,7 +19,7 @@ hotp-verification_output := \
 hotp-verification_configure := \
  INSTALL="$(INSTALL)" \
  CROSS="$(CROSS)" \
- $(CROSS_TOOLS) $(MAKE) LDFLAGS="$(INSTALL)/lib/libusb-1.0.so" GITVERSION="" LIBUSB_FLAGS="-I/usr/include/libusb-1.0" PKGCONFIG="" && $(MAKE) install INSTALL="$(INSTALL)"
+ $(CROSS_TOOLS) $(MAKE) LDFLAGS="$(INSTALL)/lib/libusb-1.0.so" GITVERSION="" LIBUSB_FLAGS="-I$(INSTALL)/include/libusb-1.0" PKGCONFIG="" && $(MAKE) install INSTALL="$(INSTALL)"
 
 hotp-verification_depends += hidapi
 modules-y += hidapi