libcurl: security bump to 7.52.0

Fixes CVE-2016-9586 - printf floating point buffer overflow

For details, see:
https://curl.haxx.se/docs/adv_20161221A.html

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libcurl/libcurl.hash

@@ -1,2 +1,2 @@
 # Locally calculated after checking pgp signature
-sha256 7f8240048907e5030f67be0a6129bc4b333783b9cca1391026d700835a788dde  curl-7.51.0.tar.bz2
+sha256 b9a2e18b4785eb75ad84598720e1559e1c53550ea011c0e00becdb94e2df5cc6  curl-7.52.0.tar.bz2

package/libcurl/libcurl.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-LIBCURL_VERSION = 7.51.0
+LIBCURL_VERSION = 7.52.0
 LIBCURL_SOURCE = curl-$(LIBCURL_VERSION).tar.bz2
 LIBCURL_SITE = https://curl.haxx.se/download
 LIBCURL_DEPENDENCIES = host-pkgconf \