[libc++][format] Fix a missing include in tests.

var-const · tru · commit 99af55f91f5d · 2023-03-24T08:44:24.000+01:00
(cherry picked from commit 78f17b2)
diff --git a/libcxx/test/support/format.functions.common.h b/libcxx/test/support/format.functions.common.h
@@ -11,6 +11,7 @@
 // Contains the common part of the formatter tests for different papers.
 
 #include <algorithm>
+#include <cctype>
 #include <charconv>
 #include <format>
 #include <ranges>
