adb: add nullability specifiers to adb_client.h.

Change-Id: I93287b876cb06d871b4330d2733ef6f43c58fbbf
diff --git a/adb/sysdeps.h b/adb/sysdeps.h
index 2190c61..a39775e 100644
--- a/adb/sysdeps.h
+++ b/adb/sysdeps.h
@@ -61,6 +61,10 @@
 
 #ifdef _WIN32
 
+// Clang-only nullability specifiers
+#define _Nonnull
+#define _Nullable
+
 #include <ctype.h>
 #include <direct.h>
 #include <dirent.h>