Restore __system_property_find_nth version

Moving this to LIBC_DEPRECATED caused compatibility
problems for some apps.

Bug: http://b/36566667
Test: launch app and check that it does not crash
Change-Id: I470f916ef818ff62ff5391b3a0c86dd44c1e8cd0
(cherry picked from commit 2167e74f168a192ca9c63abee1c03197326b83d7)
diff --git a/libc/libc.arm64.map b/libc/libc.arm64.map
index 4953380..34b8b19 100644
--- a/libc/libc.arm64.map
+++ b/libc/libc.arm64.map
@@ -132,6 +132,7 @@
     __sym_ston;
     __system_property_area_serial; # introduced=23
     __system_property_find;
+    __system_property_find_nth;
     __system_property_foreach; # introduced-arm=19 introduced-arm64=21 introduced-mips=19 introduced-mips64=21 introduced-x86=19 introduced-x86_64=21
     __system_property_get;
     __system_property_read;
@@ -1249,7 +1250,6 @@
 
 LIBC_DEPRECATED {
   global:
-    __system_property_find_nth;
     __system_property_wait_any;
 };