Add missing head sys/types.h to Volume.h

Fix compile error of build with C++

Change-Id: I2dec329d23c2187d878c1244541d627c68c09a69
Signed-off-by: sndnvaps <sndnvaps@gmail.com>
https://github.com/CyanogenMod/android_system_vold/commit/054d2fd4d1207a64454e27936d67da7c8acf0373