debian: add txt2tags to Build-Depends and run 'make html' as part of the build

This is to make sure that gf-web-api.html gets included in the .deb package
and is available locally in the GF cloud when running gf -server.

debian/rules

@@ -18,6 +18,7 @@ override_dh_auto_build:
 	cd src/runtime/java && make CFLAGS="-I$(CURDIR)/src/runtime/c -L$(CURDIR)/src/runtime/c/.libs" INSTALL_PATH=/usr/lib
 	echo LD_LIBRARY_PATH=$$LD_LIBRARY_PATH:$(CURDIR)/src/runtime/c/.libs
 	LD_LIBRARY_PATH=$$LD_LIBRARY_PATH:$(CURDIR)/src/runtime/c/.libs cabal build
+	make html
 
 override_dh_auto_clean:
 	rm -fr dist/build