[core] Added SRT_SYNC_CLOCK_TYPE preprocessor definition

[maxsharabayko]

Adds SRT_SYNC_CLOCK_TYPE preprocessor definition in sync.h so that internally SRT can distinguish clock type outside of the sync.h file.
Useful for #1876 to print in the log message the clock type used by SRT.

Partially addresses #1702

TODO

• TBD separately. API function to get clock type in runtime. Maybe int srt_clock_type()?
• Not to be done (no need so far)? Include clock type preprocessor definition into srt.h?

[maxsharabayko]

API change proposal to be moved to a separate PR to unblock other changes.

srt.h

enum SRT_SYNC_CLOCK_TYPE
{
    SRT_SYNC_CLOCK_TYPE_STDCXX_STEADY      = 0, // C++11 std::chrono::steady_clock
    SRT_SYNC_CLOCK_TYPE_GETTIME_MONOTONIC  = 1, // clock_gettime with CLOCK_MONOTONIC
    SRT_SYNC_CLOCK_TYPE_WINQPC             = 2,
    SRT_SYNC_CLOCK_TYPE_MACH_ABSTIME       = 3,
    SRT_SYNC_CLOCK_TYPE_POSIX_GETTIMEOFDAY = 4,
    SRT_SYNC_CLOCK_TYPE_AMD64_RDTSC        = 5,
    SRT_SYNC_CLOCK_TYPE_IA32_RDTSC         = 6,
    SRT_SYNC_CLOCK_TYPE_IA64_ITC           = 7
};

SRT_API int srt_clock_type();

srt_c_api.cpp

int srt_clock_type()
{
    return SRT_SYNC_CLOCK;
}

[ethouris]

Beside these names, everything looks good.