adb: fix windows test build.

The switch over to soong unintentionally turned off building the
Windows tests (and there's a bug preventing them from being turned on).
Preemptively fix the build breakages that'll happen when we turn them
back on.

Bug: http://b/74616284
Test: mma with Android.bp 's/cc_test_host/cc_binary_host' hacks
Change-Id: I6582cfc61b96052537d50d8ba90533dbb66e3e1d
(cherry picked from commit 2e1e789e2ac34402810321334537a3dd4992705c)
diff --git a/adb/Android.mk b/adb/Android.mk
index e52f0cb..c473ed2 100644
--- a/adb/Android.mk
+++ b/adb/Android.mk
@@ -36,6 +36,7 @@
 #   CreateFileW(path_wide.c_str());
 ADB_COMMON_windows_CFLAGS := \
     -DUNICODE=1 -D_UNICODE=1 \
+    -D_POSIX_SOURCE
 
 # libadb
 # =========================================================