Rename debug_stacktrace to debug_backtrace.

All the functions say "backtrace", so it's weird that the filename
says "stacktrace".

Change-Id: I1c88a56c1f9adb1bae4615edc8af3e73db270a8c
diff --git a/libc/Android.mk b/libc/Android.mk
index d6b208c..4b98ee7 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -1428,8 +1428,8 @@
     $(libc_common_c_includes) \
 
 LOCAL_SRC_FILES := \
+    bionic/debug_backtrace.cpp \
     bionic/debug_mapinfo.cpp \
-    bionic/debug_stacktrace.cpp \
     bionic/libc_logging.cpp \
     bionic/malloc_debug_leak.cpp \
     bionic/malloc_debug_check.cpp \