extern: update xmlsec to 1.2.28

Meant to fix the build problems reported in the mail thread starting at
<https://lists.freedesktop.org/archives/libreoffice/2019-March/082340.html>
"Build failure with latest ICU 64.1".

Change-Id: I006b92f4737f5e56e50527dd954e8c0d339e75dc
Reviewed-on: https://gerrit.libreoffice.org/71143
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

download.lst

@@ -154,8 +154,8 @@ export LIBNUMBERTEXT_SHA256SUM := e1c9086b4cecb6b25f180316f30740dfabe6a4dbaf70dd
 export LIBNUMBERTEXT_TARBALL := libnumbertext-1.0.5.tar.xz
 export LIBTOMMATH_SHA256SUM := 083daa92d8ee6f4af96a6143b12d7fc8fe1a547e14f862304f7281f8f7347483
 export LIBTOMMATH_TARBALL := ltm-1.0.zip
-export XMLSEC_SHA256SUM := 97d756bad8e92588e6997d2227797eaa900d05e34a426829b149f65d87118eb6
-export XMLSEC_TARBALL := xmlsec1-1.2.27.tar.gz
+export XMLSEC_SHA256SUM := 13eec4811ea30e3f0e16a734d1dbf7f9d246a71d540b48d143a07b489f6222d4
+export XMLSEC_TARBALL := xmlsec1-1.2.28.tar.gz
 export LIBXML_SHA256SUM := 94fb70890143e3c6549f265cee93ec064c80a84c42ad0f23e85ee1fd6540a871
 export LIBXML_VERSION_MICRO := 9
 export LIBXML_TARBALL := libxml2-2.9.$(LIBXML_VERSION_MICRO).tar.gz