Use -fvisibility=hidden to build libm.

Bug: 11156955
Change-Id: I77b7772a44313a1eb3970f5d2c8e2af63690efa4
diff --git a/libm/Android.mk b/libm/Android.mk
index d7d8bc1..4dfb216 100644
--- a/libm/Android.mk
+++ b/libm/Android.mk
@@ -239,6 +239,7 @@
     -Wno-sign-compare \
     -Wno-uninitialized \
     -Wno-unknown-pragmas \
+    -fvisibility=hidden \
 
 libm_common_includes := $(LOCAL_PATH)/upstream-freebsd/lib/msun/src/