Ignore windows in the test.

src/test/run-make-fulldeps/incr-add-rust-src-component/Makefile

@@ -3,6 +3,11 @@
 # rust-lang/rust#70924: Test that if we add rust-src component in between two
 # incremetnal compiles, the compiler does not ICE on the second.
 
+# This test uses `ln -s` rather than copying to save testing time, but its
+# usage doesn't work on windows. So ignore windows.
+
+# ignore-windows
+
 SYSROOT:=$(shell $(RUSTC) --print sysroot)
 FAKEROOT=$(TMPDIR)/fakeroot
 INCR=$(TMPDIR)/incr