[libc++] Granularize algorithm includes

Reviewed By: Mordante, ldionne, Quuxplusone, #libc, #libc_abi

Spies: #libc_vendors, libcxx-commits, miyuki

Differential Revision: https://reviews.llvm.org/D119667

NOKEYCHECK=True
GitOrigin-RevId: 2e2f3158c604adb8401a2a44a03f58d4b6f1c7f9
diff --git a/include/regex b/include/regex
index f2b53f1..dadd7b9 100644
--- a/include/regex
+++ b/include/regex
@@ -762,6 +762,7 @@
 } // std
 */
 
+#include <__algorithm/find.h>
 #include <__config>
 #include <__debug>
 #include <__iterator/wrap_iter.h>