util/consideredharmful: Use overloadable func attribute with clang

When building with glibc HEAD, it has fortified headers with clang as well
and clang reports errors e.g.

| ../git/src/util/consideredharmful.h:33:7: error: redeclaration of 'strncat' must have the 'overloadable' attribute
|    33 | char* strncat();
|       |       ^
| /mnt/b/yoe/master/build/tmp/work/core2-64-yoe-linux/taisei/1.4.2/recipe-sysroot/usr/include/bits/string_fortified.h:145:8: note: previous overload of function is here
|   145 | __NTH (strncat (__fortify_clang_overload_arg (char *, __restrict, __dest),
|       |        ^

src/util/consideredharmful.h

@@ -30,11 +30,11 @@ FILE* fopen();
 
 #undef strncat
 attr_deprecated("This function likely doesn't do what you expect, use strlcat")
-char* strncat();
+char* __attribute__((overloadable)) strncat();
 
 #undef strncpy
 attr_deprecated("This function likely doesn't do what you expect, use strlcpy")
-char* strncpy();
+char* __attribute__((overloadable)) strncpy();
 
 #undef errx
 attr_deprecated("Use log_fatal instead")