Update overlooked window function as part of filter speedup

src/fm_mpx.c

@@ -151,7 +151,7 @@ int fm_mpx_open(char *filename, size_t len) {
             firpreemph=y;                   // y could be replaced by firpreemph but this
                                             // matches the example in the reference material
 
-            window = (.54 - .46 * cos(2*PI * (i) / (double) FIR_TAPS*FIR_PHASES )) ; // Hamming window
+            window = (.54 - .46 * cos(2*PI * (mi) / (double) FIR_TAPS*FIR_PHASES )) ; // Hamming window
             low_pass_fir[j][i] = firpreemph * window * gain ; 
           }
         }