Cleanup libbacktrace Android.mk

Removed unnecessary libbacktrace_c_includes variable

Change-Id: I2ef29dc9810f7d9a7dcf5c232e9e85d2fc2fd5e8
diff --git a/libbacktrace/Android.mk b/libbacktrace/Android.mk
index f2f71f9..ca1e4bf 100755
--- a/libbacktrace/Android.mk
+++ b/libbacktrace/Android.mk
@@ -51,9 +51,6 @@
 	UnwindMap.cpp \
 	UnwindPtrace.cpp \
 
-libbacktrace_c_includes := \
-	external/libunwind/include \
-
 libbacktrace_shared_libraries := \
 	libunwind \
 	libunwind-ptrace \