unix/Makefile: Allow variants to add QSTR_DEFS.

This modifies the unix 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/unix/Makefile

@@ -27,8 +27,8 @@ FROZEN_MANIFEST ?= variants/manifest.py
 PROG ?= micropython
 
 # qstr definitions (must come before including py.mk)
-QSTR_DEFS = qstrdefsport.h
-QSTR_GLOBAL_DEPENDENCIES = $(VARIANT_DIR)/mpconfigvariant.h
+QSTR_DEFS += qstrdefsport.h
+QSTR_GLOBAL_DEPENDENCIES += $(VARIANT_DIR)/mpconfigvariant.h
 
 # OS name, for simple autoconfig
 UNAME_S := $(shell uname -s)