Add versioning information to symbols.

Bug: http://b/28178111
Change-Id: I46bf95accd819f4521afb1173d8badcc5e9df31c
diff --git a/libc/include/sys/system_properties.h b/libc/include/sys/system_properties.h
index 01c3db3..99a6e59 100644
--- a/libc/include/sys/system_properties.h
+++ b/libc/include/sys/system_properties.h
@@ -48,7 +48,7 @@
 
 /* Set a system property by name.
 **/
-int __system_property_set(const char *key, const char *value);
+int __system_property_set(const char* key, const char* value) __INTRODUCED_IN(21);
 
 /* Return a pointer to the system property named name, if it
 ** exists, or NULL if there is no such property.  Use 
@@ -93,9 +93,8 @@
 ** Order of results may change from call to call.  This is
 ** not a bug.
 */
-int __system_property_foreach(
-        void (*propfn)(const prop_info *pi, void *cookie),
-        void *cookie);
+int __system_property_foreach(void (*propfn)(const prop_info* pi, void* cookie), void* cookie)
+  __INTRODUCED_IN(21);
 
 __END_DECLS