gnutls: switch to https download location

HTTPS works better behind certain corporate firewalls.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 9289b2427585279d706a6a75219108d3cd0fdaba)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/gnutls/gnutls.mk

@@ -7,7 +7,7 @@
 GNUTLS_VERSION_MAJOR = 3.5
 GNUTLS_VERSION = $(GNUTLS_VERSION_MAJOR).13
 GNUTLS_SOURCE = gnutls-$(GNUTLS_VERSION).tar.xz
-GNUTLS_SITE = ftp://ftp.gnutls.org/gcrypt/gnutls/v$(GNUTLS_VERSION_MAJOR)
+GNUTLS_SITE = https://www.gnupg.org/ftp/gcrypt/gnutls/v$(GNUTLS_VERSION_MAJOR)
 GNUTLS_LICENSE = LGPL-2.1+ (core library), GPL-3.0+ (gnutls-openssl library)
 GNUTLS_LICENSE_FILES = doc/COPYING doc/COPYING.LESSER
 GNUTLS_DEPENDENCIES = host-pkgconf libunistring libtasn1 nettle pcre