Add include guards in pll.h

[amkozlov]

Including both pll.h and pll_optimize.h fails, because this check in pll_optimize.h

#ifndef PLL_H_
#define PLL_H_
#include <pll.h>
#endif

doesn't make sense (since pll_optimize.h itself is guarded), it should be moved into pll.h instead.

[xflouris]

fixed