libm: rename LIBC_PRIVATE to LIBC_DEPRECATED

Bug: http://b/26386014
Change-Id: Ia32411ce779f9dcf2cf7910d8658ffa753c73af9
diff --git a/libm/libm.arm.map b/libm/libm.arm.map
index e781f2d..842f46c 100644
--- a/libm/libm.arm.map
+++ b/libm/libm.arm.map
@@ -272,7 +272,7 @@
     *;
 };
 
-LIBC_PRIVATE { # arm mips
+LIBC_DEPRECATED { # arm mips
   global: # arm mips
     ___Unwind_Backtrace; # arm
     ___Unwind_ForcedUnwind; # arm