Update AudioMultiply_F32.h

Oops, forgot to edit the title in the comment.
Also added guard block so there will be no problem including it in multiple places (i guess all .h files should have this)

AudioMultiply_F32.h

@@ -1,5 +1,5 @@
 /*
- * AudioEffectsGain
+ * AudioMultiply
  * 
  * Created: Patrick Radius, December 2016
  * Purpose: Multiply two channels of audio data. Can be used for example as 'vca' or amplitude modulation.
@@ -9,6 +9,8 @@
  *          
  * MIT License.  use at your own risk.
 */
+#ifndef AUDIOMULTIPLYF32_H
+#define AUDIOMULTIPLYF32_H
 
 #include <arm_math.h>
 #include <AudioStream_F32.h>
@@ -22,3 +24,5 @@ class AudioMultiply_F32 : public AudioStream_F32
   private:
     audio_block_f32_t *inputQueueArray_f32[2];
 };
+
+#endif