Add versioning information to symbols.

Bug: http://b/28178111
Change-Id: I46bf95accd819f4521afb1173d8badcc5e9df31c
diff --git a/libc/include/uchar.h b/libc/include/uchar.h
index a5e72ea..9a0e2f8 100644
--- a/libc/include/uchar.h
+++ b/libc/include/uchar.h
@@ -42,16 +42,12 @@
 #define __STD_UTF_16__ 1
 #define __STD_UTF_32__ 1
 
-size_t c16rtomb(char* __restrict, char16_t, mbstate_t* __restrict);
-size_t c32rtomb(char* __restrict, char32_t, mbstate_t* __restrict);
-size_t mbrtoc16(char16_t* __restrict,
-                const char* __restrict,
-                size_t,
-                mbstate_t* __restrict);
-size_t mbrtoc32(char32_t* __restrict,
-                const char* __restrict,
-                size_t,
-                mbstate_t* __restrict);
+size_t c16rtomb(char* __restrict, char16_t, mbstate_t* __restrict) __INTRODUCED_IN(21);
+size_t c32rtomb(char* __restrict, char32_t, mbstate_t* __restrict) __INTRODUCED_IN(21);
+size_t mbrtoc16(char16_t* __restrict, const char* __restrict, size_t, mbstate_t* __restrict)
+  __INTRODUCED_IN(21);
+size_t mbrtoc32(char32_t* __restrict, const char* __restrict, size_t, mbstate_t* __restrict)
+  __INTRODUCED_IN(21);
 
 __END_DECLS