Add volatile to temporary variable.

FuseBuffer::HandleNotImpl save the value of |request.header.unique| to the
temporary variable, clear the buffer which is a union of |request| and
|response|, then write back the unique value to response.header.unique.

Before the CL, the temporary variable was wrongly removed by the compiler
optimization, and response.header.unique was always 0. The CL adds
volatile modifier as workaround to prevent the compiler optimization
from removing the temporary value.

Bug: 62429763
Test: libappfuse_tests
Change-Id: Ia853f805633f646f316f585a35c7b018000b6eb3