treewide: fix assorted issues revealed by the meta checker

Turns out a couple of the licenses were wrong, as well as being strings.

pkgs/development/libraries/libxml2/default.nix

@@ -70,7 +70,7 @@ in stdenv.mkDerivation rec {
   meta = {
     homepage = http://xmlsoft.org/;
     description = "An XML parsing library for C";
-    license = "bsd";
+    license = lib.licenses.mit;
     platforms = lib.platforms.unix;
     maintainers = [ lib.maintainers.eelco ];
   };

pkgs/development/libraries/libxslt/default.nix

@@ -48,7 +48,7 @@ stdenv.mkDerivation rec {
   meta = with stdenv.lib; {
     homepage = http://xmlsoft.org/XSLT/;
     description = "A C library and tools to do XSL transformations";
-    license = "bsd";
+    license = licenses.mit;
     platforms = platforms.unix;
     maintainers = [ maintainers.eelco ];
   };