Make widl default to prefix/include instead of prefix/target/include, for windows toolchains

In these setups, we already install headers into prefix/include, in
order to save diskspace.

build-cross-tools.sh

@@ -16,6 +16,6 @@ export HOST=$CROSS_ARCH-w64-mingw32
 
 ./build-llvm.sh $PREFIX
 ./strip-llvm.sh $PREFIX
-./build-mingw-w64-widl.sh $PREFIX
+./build-mingw-w64-widl.sh $PREFIX --skip-triplet-prefix
 ./install-wrappers.sh $PREFIX
 ./prepare-cross-toolchain.sh $NATIVE $PREFIX $CROSS_ARCH