Proactively remove libc++ __tuple header file that became a directory

This was already handled in ObsoleteFiles.inc (see the 20230622 entry),
but some people never run "make delete-old", or want to upgrade directly
from a revision that still had libc++ 15 to the most recent revision.

They would then encounter a failure during installworld, similar to:

  install: target directory `/usr/include/c++/v1/__tuple/' does not exist

Therefore, clean it up in the distrib-cleanup phase, similar to the
earlier instance of the libc++ __string header file that became a
directory.

PR:		273753
MFC after:	1 month

etc/Makefile

@@ -124,6 +124,7 @@ MTREES+=	../${mtree}			/
 # Clean up files that have changed into directories, as mtree cannot handle this
 # scenario.
 DISTRIB_CLEANUP_FILES+=	${INCLUDEDIR}/c++/v1/__string
+DISTRIB_CLEANUP_FILES+=	${INCLUDEDIR}/c++/v1/__tuple
 distrib-cleanup: .PHONY
 	for file in ${DISTRIB_CLEANUP_FILES}; do \
 		if [ -f ${DESTDIR}/$${file} ]; then \