Don't make ncdu.1 depend on Makefile

Makefile is generated by configure, and thus guaranteed to be newer than
ncdu.1, causing make to regenerate ncdu.1 regardless of whether pod2man
is installed or not. That kinda defeats the purpose of including ncdu.1
in the distribution.

Makefile.am

@@ -34,6 +34,6 @@ EXTRA_DIST=ncdu.1 doc/ncdu.pod
 
 # Don't "clean" ncdu.1, it should be in the tarball so that pod2man isn't a
 # build dependency for those who use the tarball.
-ncdu.1: $(srcdir)/doc/ncdu.pod Makefile
+ncdu.1: $(srcdir)/doc/ncdu.pod
 	pod2man --center "ncdu manual" --release "@PACKAGE@-@VERSION@" "$(srcdir)/doc/ncdu.pod" >ncdu.1