Fix windows build of Mesa.

Just adding two tiny text files here allows build-mesa.sh to work
properly when building Windows binaries with a fresh checkout of
this directory.

See https://android-review.googlesource.com/#/c/152704/ for context.

Change-Id: I9dca1d4a7be0be7dfb1565020d237dd621439fd7

windows-x86/bin/DO_NOT_REMOVE

diff --git a/windows-x86/bin/DO_NOT_REMOVE b/windows-x86/bin/DO_NOT_REMOVE
new file mode 100644
index 0000000..e886aa5
--- /dev/null
+++ b/windows-x86/bin/DO_NOT_REMOVE
@@ -0,0 +1 @@
+This file is required to build Mesa

windows-x86_64/bin/DO_NOT_REMOVE

diff --git a/windows-x86_64/bin/DO_NOT_REMOVE b/windows-x86_64/bin/DO_NOT_REMOVE
new file mode 100644
index 0000000..e886aa5
--- /dev/null
+++ b/windows-x86_64/bin/DO_NOT_REMOVE
@@ -0,0 +1 @@
+This file is required to build Mesa