mkrules: Fix warning preprocessing C++ files

Messages like 'command-line option is [not valid] for C++' can result
from the way the preprocessor is invoked by `genlast`.  Instead, cause
the files to be preprocessed as though their content is "C".  This
should generally be OK, as they'll eventually be _compiled_ as C++.
When preprocessed as C, the file simply needs to generate all the same
QSTRS and TRANSLATEs.

py/mkrules.mk

@@ -65,7 +65,7 @@ vpath %.cpp . $(TOP) $(USER_C_MODULES)
 $(BUILD)/%.o: %.cpp
 	$(call compile_cxx)
 
-QSTR_GEN_EXTRA_CFLAGS += -DNO_QSTR
+QSTR_GEN_EXTRA_CFLAGS += -DNO_QSTR -x c
 
 # frozen.c and frozen_mpy.c are created in $(BUILD), so use our rule
 # for those as well.