Fix libbacktrace host build.

On host build, libbacktrace relies on libcutils to provide gettid()
function.

Change-Id: Iad83384ea699ca024b71b73cef26755287c9e6fe
diff --git a/libbacktrace/Android.mk b/libbacktrace/Android.mk
index 9c6742e..5d3dd86 100644
--- a/libbacktrace/Android.mk
+++ b/libbacktrace/Android.mk
@@ -68,6 +68,7 @@
 # Use static llvm libraries on host to remove dependency on 32-bit llvm shared library
 # which is not included in the prebuilt.
 libbacktrace_static_libraries_host := \
+	libcutils \
 	libLLVMObject \
 	libLLVMBitReader \
 	libLLVMMC \