py/mkrules.mk: Force ".pp" files to always rebuild.

These files are only built on demand for developers, and it is a quick
process.

Without FORCE, a sequence like this would leave the developer with an
outdated `main.pp` to inspect:

    make build-standard/main.pp
    touch input.h
    make build-standard/main.pp # Rebuilds now, wouldn't have before

Signed-off-by: Jeff Epler <jepler@gmail.com>

py/mkrules.mk

@@ -104,7 +104,7 @@ vpath %.cpp . $(TOP) $(USER_C_MODULES)
 $(BUILD)/%.o: %.cpp
 	$(call compile_cxx)
 
-$(BUILD)/%.pp: %.c
+$(BUILD)/%.pp: %.c FORCE
 	$(ECHO) "PreProcess $<"
 	$(Q)$(CPP) $(CFLAGS) -Wp,-C,-dD,-dI -o $@ $<