libgcrypt: specify where gpg-error-config is

libgcrypt depends on libgpg-error, but it needs to know where the
gpg-error-config utility is.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libgcrypt/libgcrypt.mk

@@ -12,7 +12,8 @@ LIBGCRYPT_INSTALL_TARGET = YES
 LIBGCRYPT_CONF_ENV = \
 	ac_cv_sys_symbol_underscore=no
 LIBGCRYPT_CONF_OPT = \
-	--disable-optimization
+	--disable-optimization \
+	--with-gpg-error-prefix=$(STAGING_DIR)/usr
 
 LIBGCRYPT_DEPENDENCIES = libgpg-error