libcurl: security bump to version 7.57.0

Fixes the following security issues:

- CVE-2017-8816: NTLM buffer overflow via integer overflow
- CVE-2017-8817: FTP wildcard out of bounds read
- CVE-2017-8818: SSL out of buffer access

For more details, see the changelog:
https://curl.haxx.se/changes.html#7_57_0

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

package/libcurl/libcurl.hash

@@ -1,4 +1,4 @@
 # Locally calculated after checking pgp signature
-# https://curl.haxx.se/download/curl-7.56.1.tar.xz.asc
-sha256 8eed282cf3a0158d567a0feaa3c4619e8e847970597b5a2c81879e8f0d1a39d1  curl-7.56.1.tar.xz
+# https://curl.haxx.se/download/curl-7.57.0.tar.xz.asc
+sha256 f5f6fd3c72b7b8389969f4fb671ed8532fa9b5bb7a5cae7ca89bc1cea45c7878  curl-7.57.0.tar.xz
 sha256 cbcf511f5702f7baf5424193a792bc9c18fab22bcbec2e6a587598389dc632c2  COPYING

package/libcurl/libcurl.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-LIBCURL_VERSION = 7.56.1
+LIBCURL_VERSION = 7.57.0
 LIBCURL_SOURCE = curl-$(LIBCURL_VERSION).tar.xz
 LIBCURL_SITE = https://curl.haxx.se/download
 LIBCURL_DEPENDENCIES = host-pkgconf \