Added audio capture support to SDL backend.

This lets you speak through VoIP when not using OpenAL. Previously you could
listen but not speak.

code/null/null_snddma.c

@@ -48,6 +48,30 @@ void SNDDMA_Submit(void)
 {
 }
 
+#ifdef USE_VOIP
+void SNDDMA_StartCapture(void)
+{
+}
+
+int SNDDMA_AvailableCaptureSamples(void)
+{
+	return 0;
+}
+
+void SNDDMA_Capture(int samples, byte *data)
+{
+}
+
+void SNDDMA_StopCapture(void)
+{
+}
+
+void SNDDMA_MasterGain( float val )
+{
+}
+#endif
+
+
 sfxHandle_t S_RegisterSound( const char *name, qboolean compressed ) 
 {
 	return 0;