package/gnupg2: fix libusb detection

gnupg2 does not use pkgconf to detect the include dir of libusb.

Fixes
http://autobuild.buildroot.net/results/70c/70c9f5f8ae77fbda2b5eba3aacf831430dc59a1c/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/gnupg2/gnupg2.mk

@@ -43,6 +43,7 @@ GNUPG2_CONF_OPTS += --disable-gnutls
 endif
 
 ifeq ($(BR2_PACKAGE_LIBUSB_COMPAT),y)
+GNUPG2_CONF_ENV += CPPFLAGS="$(TARGET_CPPFLAGS) -I$(STAGING_DIR)/usr/include/libusb-1.0"
 GNUPG2_CONF_OPTS += --enable-ccid-driver
 GNUPG2_DEPENDENCIES += libusb-compat
 else