gbuild: fix dependencies in gb_SrsPartTarget__command_dep

Use -MP to output dummy rules for included files so build doesn't break
when an included file is removed.

Change-Id: Ib230cfa5d5da71c7f5c15af5bdaa5004e6b7a122

solenv/gbuild/platform/com_GCC_class.mk

@@ -163,7 +163,7 @@ define gb_SrsPartTarget__command_dep
 $(call gb_Helper_abbreviate_dirs,\
 	mkdir -p $(dir $(call gb_SrsPartTarget_get_dep_target,$(1))) && cd $(SRCDIR) && \
 	$(gb_GCCP) \
-		-MM -MT $(call gb_SrsPartTarget_get_target,$(1)) \
+		-MM -MP -MT $(call gb_SrsPartTarget_get_target,$(1)) \
 		$(INCLUDE) \
 		$(DEFS) \
 		-c -x c++-header $(2) \