Merge pull request #6852 from tcm0116/mixing_extruder_lin_advance

Generate error if MIXING_EXTRUDER and LIN_ADVANCE are both enabled

Marlin/SanityCheck.h

     #error "Please select either MIXING_EXTRUDER or SWITCHING_EXTRUDER, not both."
   #elif ENABLED(SINGLENOZZLE)
     #error "MIXING_EXTRUDER is incompatible with SINGLENOZZLE."
+  #elif ENABLED(LIN_ADVANCE)
+    #error "MIXING_EXTRUDER is incompatible with LIN_ADVANCE."
   #endif
 #endif