dri: Ensure that DRI driver cpp files are in tarballs

Makefile

@@ -356,6 +356,7 @@ DRI_FILES = \
 	$(DIRECTORY)/src/mesa/drivers/dri/common/xmlpool/*.[ch]		\
 	$(DIRECTORY)/src/mesa/drivers/dri/common/xmlpool/*.po		\
 	$(DIRECTORY)/src/mesa/drivers/dri/*/*.[chS]			\
+	$(DIRECTORY)/src/mesa/drivers/dri/*/*.cpp			\
 	$(DIRECTORY)/src/mesa/drivers/dri/*/*/*.[chS]			\
 	$(DIRECTORY)/src/mesa/drivers/dri/*/Makefile			\
 	$(DIRECTORY)/src/mesa/drivers/dri/*/*/Makefile			\