Make d_H inherited from base class.

All equalizer classes had their own private `d_H` vector, despite all being derived from a common `base` class. Now `d_H` is instead a protected member of `base`, and thus accessible to the derived equalizer classes.

lib/equalizer/base.h

@@ -35,6 +35,8 @@ public:
 
 protected:
 	static const gr_complex LONG[64];
+
+	gr_complex d_H[64];
 };
 
 } /* namespace channel_estimation */