docs: preserve the modification date when copying the prebuilt man page

The previously built man page "curl.1" must be copied with the original
modification date, otherwise the man page is never updated.

This fixes a bug that has been introduced with commit 2568441cab.

Reviewed-by: Dan Fandrich
Reviewed-by: Daniel Stenberg

Closes #12199

docs/Makefile.am

@@ -111,7 +111,8 @@ SUFFIXES = .1 .html .pdf
 # have changed.
 $(abs_builddir)/curl.1:
 	if test "$(top_builddir)x" != "$(top_srcdir)x" -a -e "$(srcdir)/curl.1"; then \
-		$(INSTALL_DATA) "$(srcdir)/curl.1" $@; fi
+		$(INSTALL_DATA) "$(srcdir)/curl.1" $@ \
+		&& touch -r "$(srcdir)/curl.1" $@; fi
 	cd cmdline-opts && $(MAKE)
 
 html: $(HTMLPAGES)