fixed a small precompiler flag issue with the magnetic signal velocity,...

fixed a small precompiler flag issue with the magnetic signal velocity, confirmed that the code doesn't crash becasue of the definition of magnetic signal velocity so it must be somethig more fundamental

src/data/allvars.h

@@ -24,11 +24,16 @@
 #include "../io/parameters.h"
 
 #ifdef MHD
+
+#ifndef MAGNETIC_SIGNALVEL
 #define MAGNETIC_SIGNALVEL
+#endif
+
 #ifndef DIVBFORCE 
 #define DIVBFORCE 1
 #endif
-#endif
+
+#endif //MHD
 
 /** Data which is the SAME for all tasks (mostly code parameters read
  * from the parameter file).  Holding this data in a structure is