[lldb] Make sure TestDAP_subtleFrames actually uses libc++ (#108227)

Without this, the binary would still be built with libstdc++, if that's
the system/compiler default.

Author: labath

lldb/test/API/tools/lldb-dap/stackTrace/subtleFrames/Makefile

@@ -1,3 +1,4 @@
 CXX_SOURCES := main.cpp
+USE_LIBCPP := 1
 
 include Makefile.rules