fix WinRPM package name

see https://build.opensuse.org/package/rdiff/windows:mingw:win64/mingw64-libxml2?linkrev=base&rev=29, dll was moved into a separately named package

Author: tkelman

deps/build.jl

@@ -5,7 +5,7 @@ using BinDeps
 
 libxml2 = library_dependency("libxml2-2")
 using WinRPM
-provides(WinRPM.RPM, "libxml2", libxml2, os = :Windows)
+provides(WinRPM.RPM, "libxml2-2", libxml2, os = :Windows)
 
 @BinDeps.install
 end