stm32/Makefile: Add missing ordering dependency on generated headers.

ports/stm32/Makefile

@@ -14,9 +14,12 @@ include ../../py/mkenv.mk
 -include mpconfigport.mk
 include $(BOARD_DIR)/mpconfigboard.mk
 
+# Files that are generated and needed before the QSTR build.
+QSTR_GENERATED_HEADERS = $(BUILD)/pins_qstr.h $(BUILD)/modstm_qstr.h
+
 # qstr definitions (must come before including py.mk)
-QSTR_DEFS = qstrdefsport.h $(BUILD)/pins_qstr.h $(BUILD)/modstm_qstr.h
-QSTR_GLOBAL_DEPENDENCIES = mpconfigboard_common.h $(BOARD_DIR)/mpconfigboard.h
+QSTR_DEFS = qstrdefsport.h $(QSTR_GENERATED_HEADERS)
+QSTR_GLOBAL_DEPENDENCIES = mpconfigboard_common.h $(BOARD_DIR)/mpconfigboard.h $(QSTR_GENERATED_HEADERS)
 
 # File containing description of content to be frozen into firmware.
 FROZEN_MANIFEST ?= boards/manifest.py