py/mkrules.mk: Remove unnecessary ; in makefile.

This ; make Windows compilation fail with GNU makefile 4.2.1.  It was added
in 0dc85c9f86 as part of a shell if-
statement, but this if-statement was subsequently removed in
23a693ec2d so the semicolon is not needed.

py/mkrules.mk

@@ -72,7 +72,7 @@ $(OBJ): | $(HEADER_BUILD)/qstrdefs.generated.h $(HEADER_BUILD)/mpversion.h
 # - else, process all source files ($^) [this covers "make -B" which can set $? to empty]
 $(HEADER_BUILD)/qstr.i.last: $(SRC_QSTR) $(QSTR_GLOBAL_DEPENDENCIES) | $(HEADER_BUILD)/mpversion.h
 	$(ECHO) "GEN $@"
-	$(Q)$(CPP) $(QSTR_GEN_EXTRA_CFLAGS) $(CFLAGS) $(if $(filter $?,$(QSTR_GLOBAL_DEPENDENCIES)),$^,$(if $?,$?,$^)) >$(HEADER_BUILD)/qstr.i.last;
+	$(Q)$(CPP) $(QSTR_GEN_EXTRA_CFLAGS) $(CFLAGS) $(if $(filter $?,$(QSTR_GLOBAL_DEPENDENCIES)),$^,$(if $?,$?,$^)) >$(HEADER_BUILD)/qstr.i.last
 
 $(HEADER_BUILD)/qstr.split: $(HEADER_BUILD)/qstr.i.last
 	$(ECHO) "GEN $@"