tools/expat: change package source to GitHub

According to the SourceForge page, the project is migrating to GitHub.
Thus, change the source of the package to GitHub.

Signed-off-by: David Bauer <mail@david-bauer.net>

tools/expat/Makefile

@@ -13,7 +13,7 @@ PKG_VERSION:=2.2.9
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.bz2
 PKG_HASH:=f1063084dc4302a427dabcca499c8312b3a32a29b7d2506653ecc8f950a9a237
-PKG_SOURCE_URL:=@SF/expat
+PKG_SOURCE_URL:=https://github.com/libexpat/libexpat/releases/download/R_$(subst .,_,$(PKG_VERSION))
 
 HOST_BUILD_PARALLEL:=1