utils: fix broken stopwatch implementation

Stopwatch (in the way it is used) must be able to measure the wall
clock time with high resolution but used clock() which is not
appropriate for this purpose:
On POSIX systems clock() returns the processor time used by the
program. On Windows clock() does measure the wall clock time but
has only a resolution of 1ms (if at all).
This also renders the freerdp profiler unusable.

This commit changes stopwatch to use the performance counters
on Windows and gettimeofday() for the rest.

Also added a warning about invalid profiling results to the
RemoteFX codec if rfxcontext->priv->UseThreads is enabled because
stopwatch is currently not used in a thread safe way.

Also see GitHub Issue #1325