Shush a MinGW warning in crypto/x509.

The MinGW setup on Android already defines this stat macro.

Change-Id: Ia8e89195c06ec01d4b5a2fa7357fb8d2d500aa06
Reviewed-on: https://boringssl-review.googlesource.com/11124
Reviewed-by: Kenny Root <kroot@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/x509/by_dir.c

diff --git a/crypto/x509/by_dir.c b/crypto/x509/by_dir.c
index 4445b05..434e5ab 100644
--- a/crypto/x509/by_dir.c
+++ b/crypto/x509/by_dir.c
@@ -362,7 +362,7 @@
                                  postfix, k);
                 }
 #ifndef OPENSSL_NO_POSIX_IO
-# ifdef _WIN32
+# if defined(_WIN32) && !defined(stat)
 #  define stat _stat
 # endif
                 {