Add libm due to libpng dependency.

Change-Id: I0bdc2df5ef358813587f613a1b50eaa850e95782
diff --git a/Android.mk b/Android.mk
index 1c78b42..215cfe5 100644
--- a/Android.mk
+++ b/Android.mk
@@ -47,6 +47,7 @@
     libpng \
     libcutils \
     libstdc++ \
+    libm \
     libc
 
 ifeq ($(TARGET_USERIMAGES_USE_EXT4), true)