[libc++][nfc] Improve standard conformance.

The return type of the deleted functions doesn't match the synopsis in
the standard.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D114000

GitOrigin-RevId: bfc253c00068a5c5252647b6a09aef7b5f232501
diff --git a/include/charconv b/include/charconv
index 447cef9..c0e43e5 100644
--- a/include/charconv
+++ b/include/charconv
@@ -107,8 +107,8 @@
 
 #ifndef _LIBCPP_CXX03_LANG
 
-void to_chars(char*, char*, bool, int = 10) = delete;
-void from_chars(const char*, const char*, bool, int = 10) = delete;
+to_chars_result to_chars(char*, char*, bool, int = 10) = delete;
+from_chars_result from_chars(const char*, const char*, bool, int = 10) = delete;
 
 namespace __itoa
 {