[bro] fix missing malloc.h on OSX

Hello,

the sample "bro" tool does not compile on OSX (I'm using 10.9.5):

    bro.cc:19:10: fatal error: 'malloc.h' file not found

It seems like the malloc.h header file is located inside `/usr/include/malloc/malloc.h` on that platform.
With this patch, it compiles correctly.

All best,

Cosimo