logd: missing include for string.h

LogCommand.cpp gets string.h inherited from
private/android_filesystem_config.h it should
not rely on this in the future. The intent is
to move fs_config function into libcutils and
thus deprecate any need for string.h in this
include file.

Bug: 19908228
Change-Id: Iaf3a77298b56efd8004300b17e9a1faafad5b08b