estat displays CC_USING_FENTRY warnings on startup

[sebroy]

On master, when starting most of our estat scripts, the following output is emitted:

delphix@guild:~$ sudo estat backend-io 10
In file included from <built-in>:3:
/virtual/include/bcc/helpers.h:47:9: warning: 'CC_USING_FENTRY' macro redefined [-Wmacro-redefined]
#define CC_USING_FENTRY
        ^
<command line>:6:9: note: previous definition is here
#define CC_USING_FENTRY 1
        ^
1 warning generated.
...