Create base voice class

Allows us to use a single base class in audio examples instead of a specific module
Need to update name at some point as part of #9, it's not clear if UBX_CELL_VOICE or UBX_CELL_VOICE_BASE should be used based on the names alone

Author: sfe-SparkFro

src/SparkFun_u-blox_Cellular_Arduino_Library.h

@@ -1,3 +1,4 @@
 #include "sfe_ublox_cellular.h"
+#include "sfe_ublox_cellular_voice.h"
 #include "sfe_sara_r5.h"
 #include "sfe_lara_r6.h"

src/sfe_ublox_cellular_voice.h

@@ -127,4 +127,9 @@ class UBX_CELL_VOICE
   }
 };
 
+class UBX_CELL_VOICE_BASE : public UBX_CELL, public UBX_CELL_VOICE<UBX_CELL_VOICE_BASE>
+{
+
+};
+
 #endif