Add missing libm introduced=26 annotations.

Without this, the NDK stub libm.so files think we've had the new O
functions since the beginning of time.

Bug: N/A
Test: nm
Change-Id: Ibdd7b910934b706348cc6d10b2b6b63a283d7320
diff --git a/libm/libm.map.txt b/libm/libm.map.txt
index 1fb16b7..1f9a3f9 100644
--- a/libm/libm.map.txt
+++ b/libm/libm.map.txt
@@ -271,7 +271,7 @@
     *;
 };
 
-LIBC_O {
+LIBC_O { # introduced=O
   global:
     cacoshl;
     cacosl;