Fix export define for Windows

[paullouisageneau]

This PR fixes the export define for Windows, as previously it would lead to incorrect behavior, for instance re-exported symbols when used as a static library in a dynamic library.

[Nemirtingas]

Should apply the same to

libdatachannel/include/rtc/common.hpp

Lines 30 to 40 in b0a5d99

	#ifdef _WIN32
	#define RTC_CPP_EXPORT __declspec(dllexport)
	#ifndef _WIN32_WINNT
	#define _WIN32_WINNT 0x0602 // Windows 8
	#endif
	#ifdef _MSC_VER
	#pragma warning(disable : 4251) // disable "X needs to have dll-interface..."
	#endif
	#else
	#define RTC_CPP_EXPORT
	#endif

[paullouisageneau]

@Nemirtingas I fixed the C++ exports too.

[Nemirtingas]

LGTM, thanks.