windows/Makefile: Allow variants to add QSTR_DEFS.

This modifies the windows port Makefile to use += for QSTR_DEFS and
QSTR_GLOBAL_DEPENDENCIES so that variants can add additional files if
needed (similar to stm32 port).

Signed-off-by: David Lechner <david@pybricks.com>

ports/windows/Makefile

@@ -26,8 +26,8 @@ FROZEN_MANIFEST ?= variants/manifest.py
 PROG ?= micropython
 
 # qstr definitions (must come before including py.mk)
-QSTR_DEFS = ../unix/qstrdefsport.h
-QSTR_GLOBAL_DEPENDENCIES = $(VARIANT_DIR)/mpconfigvariant.h
+QSTR_DEFS += ../unix/qstrdefsport.h
+QSTR_GLOBAL_DEPENDENCIES += $(VARIANT_DIR)/mpconfigvariant.h
 
 # include py core make definitions
 include $(TOP)/py/py.mk