tools/power turbostat: Fix file pointer leak

In the Linux kernel, the following vulnerability has been resolved:

tools/power turbostat: Fix file pointer leak

Currently if a fscanf fails then an early return leaks an open file pointer. Fix this by fclosing the file before the return. Detected using static analysis with cppcheck:

tools/power/x86/turbostat/turbostat.c:2039:3: error: Resource leak: fp [resourceLeak]