zephyr: Remove reference to syscall_macros_h_target.

Zephyr removed the build target syscall_macros_h_target in commit
f4adf107f31674eb20881531900ff092cc40c07f.  Removes reference from
MicroPython to fix build errors in the zephyr port.

This change is not compatible with zephyr v2.1 or earlier.  It will be
compatible with Zephyr v2.2 when released.

ports/zephyr/Makefile

@@ -108,4 +108,4 @@ outdir/$(BOARD)/Makefile: $(CONF_FILE)
 
 $(Z_EXPORTS): outdir/$(BOARD)/Makefile
 	make --no-print-directory -C outdir/$(BOARD) outputexports CMAKE_COMMAND=: >$@
-	make -C outdir/$(BOARD) syscall_macros_h_target syscall_list_h_target kobj_types_h_target
+	make -C outdir/$(BOARD) syscall_list_h_target kobj_types_h_target