Target an older version of Windows SDK so that we don't get a conflicting definition of struct pollfd

Windows Vista and later SDKs define struct pollfd for WSAPoll(), but it has a permanent bug:
- https://curl.haxx.se/mail/lib-2012-08/0014.html
- https://curl.haxx.se/mail/lib-2012-10/0038.html
civetweb provides its own implementation of poll, but it has a conflicting definition for pollfd.
Hence we block Windows from defining pollfd (which this project doesn't use).
---
 bazel/civetweb.BUILD | 1 +
 1 file changed, 1 insertion(+)

diff --git bazel/civetweb.BUILD bazel/civetweb.BUILD
--- bazel/civetweb.BUILD
+++ bazel/civetweb.BUILD
@@ -34,5 +34,6 @@ cc_library(
         "-DNO_CACHING",
         "-DNO_SSL",
         "-DNO_FILES",
+        "-D_WIN32_WINNT=0x0502",
         "-UDEBUG",
     ],
--