Protect the container_of #define.

MOFED apparently has a /usr/include/infiniband/verbs.h that also
defines a (slightly different but fully compatible) container_of
macro.  So put proper #ifndef protection around our definition of
container_of.

Thanks to Rolf vandeVaart for pointing out the issue.

Reviewed by Dave Goodell.

cmr=v1.7.4:reviewer=ompi-rm1.7

This commit was SVN r29799.

ompi/mca/btl/usnic/btl_usnic.h

@@ -56,8 +56,10 @@ get_nsec(void)
     return usnic_ticks;
 }
 
+#ifndef container_of
 #define container_of(ptr, type, member) ( \
         (type *)( ((char *)(ptr)) - offsetof(type,member) ))
+#endif
 
 /* MSGDEBUG2 prints 1 line at each BTL entry point */
 #define MSGDEBUG2 (MSGDEBUG1||0)