Add version information for legacy inlines.

Bug: http://b/30170081
Change-Id: Ib5962ea3037dd692a98b6691d263871549aac8af
diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index 25f0304..c45a2e6 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -209,7 +209,7 @@
 int acct(const char* __filepath);
 
 #if __ANDROID_API__ >= 21
-int getpagesize(void);
+int getpagesize(void) __INTRODUCED_IN(21);
 #else
 __inline__ int getpagesize(void) {
   return sysconf(_SC_PAGESIZE);