build: don't use a dummy dependency, as it breaks non-GNU makes

This fixes https://savannah.gnu.org/bugs/?50956.
Reported-by: John Wiersba <jrw32982@yahoo.com>

src/Makefile.am

@@ -12,13 +12,11 @@ endif
 nano.o: revision.h
 winio.o: revision.h
 
-revision.h: update_revision
+revision.h:
 	@[ -f $@ ] || touch $@
 	@echo "#define $(SOMETHING)" | cmp -s $@ - || \
 	echo "#define $(SOMETHING)" > $@
 
-.PHONY: update_revision
-
 bin_PROGRAMS = 	nano
 nano_SOURCES =	browser.c \
 		chars.c \