gnutls: security bump to version 3.3.15

Fixes:
GNUTLS-SA-2015-2 - Fix for MD5 downgrade in TLS 1.2 signatures.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/gnutls/gnutls.hash

@@ -1,2 +1,2 @@
 # Locally calculated after checking pgp signature
-sha256	0dfa0030faad8909c1e904105198232d6bc0123cae8cf4933b2bac85ee7cec52	gnutls-3.3.14.tar.xz
+sha256	8961227852911a1974e15bc017ddbcd4779876c867226d199f06648d8b27ba4b	gnutls-3.3.15.tar.xz

package/gnutls/gnutls.mk

@@ -5,7 +5,7 @@
 ################################################################################
 
 GNUTLS_VERSION_MAJOR = 3.3
-GNUTLS_VERSION = $(GNUTLS_VERSION_MAJOR).14
+GNUTLS_VERSION = $(GNUTLS_VERSION_MAJOR).15
 GNUTLS_SOURCE = gnutls-$(GNUTLS_VERSION).tar.xz
 GNUTLS_SITE = ftp://ftp.gnutls.org/gcrypt/gnutls/v$(GNUTLS_VERSION_MAJOR)
 GNUTLS_LICENSE = GPLv3+ LGPLv2.1+