tools: Fix pthread flag for Makefile of trace-agent used by virtio-trace

pthread flag should not be -lpthread but -pthread using gcc. The -lpthread
links the external multithread library. On the other hand, the -pthread manages
both the gcc's preprocessor and linker to be able to compile with pthread.

Signed-off-by: Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>