libstdc++: Add new C++20 headers to Doxygen settings

This doesn't actually have any effect unless you also change the
predefined value of __cplusplus, as it's currently 201703L. But if
somebody does want to do that, the new headers will get processed now.

libstdc++-v3/ChangeLog:

	* doc/doxygen/user.cfg.in (INPUT): Add <latch> and <semaphore>.

libstdc++-v3/doc/doxygen/user.cfg.in

@@ -870,6 +870,7 @@ INPUT                  = @srcdir@/doc/doxygen/doxygroups.cc \
                          include/iostream \
                          include/istream \
                          include/iterator \
+                         include/latch \
                          include/limits \
                          include/list \
                          include/locale \
@@ -887,6 +888,7 @@ INPUT                  = @srcdir@/doc/doxygen/doxygroups.cc \
                          include/ratio \
                          include/regex \
                          include/scoped_allocator \
+                         include/semaphore \
                          include/set \
                          include/shared_mutex \
                          include/span \