selftests: vDSO: don't include generated headers for chacha test

It's not correct to use $(top_srcdir) for generated header files, for
builds that are done out of tree via O=, and $(objtree) isn't valid in
the selftests context. Instead, just obviate the need for these
generated header files by defining empty stubs in tools/include, which
is the same thing that's done for rwlock.h.
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tools/testing/selftests/vDSO/Makefile

@@ -39,7 +39,7 @@ $(OUTPUT)/vdso_test_getrandom: CFLAGS += -isystem $(top_srcdir)/tools/include \
 
 $(OUTPUT)/vdso_test_chacha: $(top_srcdir)/tools/arch/$(SRCARCH)/vdso/vgetrandom-chacha.S
 $(OUTPUT)/vdso_test_chacha: CFLAGS += -idirafter $(top_srcdir)/tools/include \
+                                      -idirafter $(top_srcdir)/tools/include/generated \
                                       -idirafter $(top_srcdir)/arch/$(SRCARCH)/include \
-                                      -idirafter $(top_srcdir)/arch/$(SRCARCH)/include/generated \
                                       -idirafter $(top_srcdir)/include \
                                       -D__ASSEMBLY__ -Wa,--noexecstack