Add missing header for EBUSY on mingw

The `error: use of undeclared identifier 'EBUSY'` in
vpx_util/vpx_pthread.h was found in Mozilla's bug 1886318 [1]. This
patch addresses the issue by adding the `<errno.h>` header to introduce
the `EBUSY` identifier, resolving the problem.

This patch is based on the change in libvpx:
0752960c6 Add missing header for EBUSY on mingw
https://chromium-review.googlesource.com/c/webm/libvpx/+/5425372

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1886318#c1

Change-Id: I90bd6b605613094fe2decfa48831f98941108a6c
(cherry picked from commit 19924274ab14db1a1132a8128e5ae7b1227743c5)

aom_util/aom_pthread.h

diff --git a/aom_util/aom_pthread.h b/aom_util/aom_pthread.h
index 827b9c2..1a97a0a 100644
--- a/aom_util/aom_pthread.h
+++ b/aom_util/aom_pthread.h
@@ -28,6 +28,7 @@
 #define NOMINMAX
 #undef WIN32_LEAN_AND_MEAN
 #define WIN32_LEAN_AND_MEAN
+#include <errno.h>    // NOLINT
 #include <process.h>  // NOLINT
 #include <stddef.h>   // NOLINT
 #include <windows.h>  // NOLINT