[adb] generic "cast_handle_to_int" function

Now one can use it without platform #ifdefs

Bug: 150183149
Test: compiles
Change-Id: I6280dcdd2faaff33014f11237d9d04231ac16b82
diff --git a/adb/sysdeps.h b/adb/sysdeps.h
index 3e781b8..9a879b5 100644
--- a/adb/sysdeps.h
+++ b/adb/sysdeps.h
@@ -665,6 +665,10 @@
     return fd.get();
 }
 
+static __inline__ int cast_handle_to_int(int fd) {
+    return fd;
+}
+
 // A very simple wrapper over a launched child process
 class Process {
   public: