<xtimec.h>: Stop exporting internal-only functions

[StephanTLavavej]

STL/stl/inc/xtimec.h

Lines 28 to 37 in f9b1dcc

	_CRTIMP2_PURE int __cdecl xtime_get(xtime*, int);
	_CRTIMP2_PURE long __cdecl _Xtime_diff_to_millis(const xtime*);
	_CRTIMP2_PURE long __cdecl _Xtime_diff_to_millis2(const xtime*, const xtime*);
	_CRTIMP2_PURE long long __cdecl _Xtime_get_ticks();
	#define _XTIME_NSECS_PER_TICK 100
	#define _XTIME_TICKS_PER_TIME_T 10000000LL
	_CRTIMP2_PURE long long __cdecl _Query_perf_counter();
	_CRTIMP2_PURE long long __cdecl _Query_perf_frequency();

Of these functions, xtime_get() and _Xtime_diff_to_millis2() are used within src only; there's no need for them to be exported.

Furthermore, _Xtime_diff_to_millis() is completely unused and should be removed.

Changing the export surface is a binary compatibility break, so this is a vNext issue.

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.