add libsurfaceflinger_ddmconnection for PDK build

- the library is dlopened from libsurfaceflinger
- the library built only when libnativehelper exists

Bug: 7089510
Change-Id: Ib3ea1029d7e8f6e055f4b759d0bf68f5123fa8a1
diff --git a/services/surfaceflinger/DdmConnection.h b/services/surfaceflinger/DdmConnection.h
index 91b737c..b6b088b 100644
--- a/services/surfaceflinger/DdmConnection.h
+++ b/services/surfaceflinger/DdmConnection.h
@@ -19,6 +19,9 @@
 
 namespace android {
 
+// wrapper for dlsym
+extern "C" void DdmConnection_start(const char* name);
+
 class DdmConnection {
 public:
     static void start(const char* name);