Conditionalize include of string.h in rx_stats.c

Conditionalize include of string.h in rx_stats.c

Linux doesn't have <string.h>, it's in the linux subdirectory.  But
it is already included by sysincludes.h, so it should not be needed
here.

Change-Id: I017c374e7b6d47e900ec73b40a36183a3f23f35c
Reviewed-on: http://gerrit.openafs.org/2888
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>