[newlib-cygwin] Cygwin: Remove intro2man.stamp on clean

[Jon TURNEY]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=174268f4bba2975276dce5582f4d3e9f4b2bdfe2

commit 174268f4bba2975276dce5582f4d3e9f4b2bdfe2
Author: Jon Turney <jon.turney@dronecode.org.uk>
Date:   Wed Oct 21 18:26:58 2020 +0100

    Cygwin: Remove intro2man.stamp on clean

Diff:
---
 winsup/doc/Makefile.in | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/winsup/doc/Makefile.in b/winsup/doc/Makefile.in
index bcf0c1fb2..cb46a91cf 100644
--- a/winsup/doc/Makefile.in
+++ b/winsup/doc/Makefile.in
@@ -61,8 +61,9 @@ clean:
 	rm -f Makefile.dep
 	rm -f *.html *.html.gz
 	rm -Rf cygwin-api cygwin-ug cygwin-ug-net faq
-	rm -f *.1 utils2man.stamp
-	rm -f *.3 api2man.stamp
+	rm -f api2man.stamp intro2man.stamp utils2man.stamp
+	rm -f *.1
+	rm -f *.3
 	rm -f *.info* charmap
 
 install: install-all