Don't distribute varnishstat_curses_help.c

It's cheap to rebuild, and it might lead to rebuilding it in $(srcdir)
instead of $(builddir) in a VPATH build. If the source tree is write
protected this will obviously fail.

Reported by phk.

bin/varnishstat/Makefile.am

@@ -11,10 +11,12 @@ varnishstat_SOURCES = \
 	varnishstat.c \
 	varnishstat_bindings.h \
 	varnishstat_curses.c \
-	varnishstat_curses_help.c \
 	varnishstat_options.h
 
-BUILT_SOURCES = varnishstat_curses_help.c
+nodist_varnishstat_SOURCES = \
+	varnishstat_curses_help.c
+
+BUILT_SOURCES = $(nodist_varnishstat_SOURCES)
 
 varnishstat_help_gen_SOURCES = \
 	varnishstat_help_gen.c \