libbacktrace: correctly number frames when skipping.

Correct for the number of skipped frames when unwinding with
libunwindstack.

Test: backtrace_test32  --gtest_filter="unwind_frame_skip_*"
Change-Id: I9528977104fde3c4ec792a6db1ada24ed571b867
diff --git a/libbacktrace/UnwindStack.cpp b/libbacktrace/UnwindStack.cpp
index c3f08c8..62e453f 100644
--- a/libbacktrace/UnwindStack.cpp
+++ b/libbacktrace/UnwindStack.cpp
@@ -64,7 +64,7 @@
     auto frame = &unwinder_frames[i];
     backtrace_frame_data_t* back_frame = &frames->at(cur_frame);
 
-    back_frame->num = frame->num;
+    back_frame->num = frame->num - num_ignore_frames;
 
     back_frame->rel_pc = frame->rel_pc;
     back_frame->pc = frame->pc;