Fix build error 'ambiguous symbol' on windows

[GkvJwa]

If use a local namespace, the following build error will occur on windows

31>C:\tdesktop\Telegram\SourceFiles\settings\settings_calls.cpp(415,30): error C2872: 'Platform': ambiguous symbol
31>(compiling source file '../../Telegram/SourceFiles/settings/settings_calls.cpp')
31>    C:\tdesktop\Telegram\SourceFiles\calls\calls_instance.h(16,11):
31>    could be 'Platform'
31>    C:\tdesktop\Telegram\lib_webrtc\webrtc\webrtc_environment.h(15,11):
31>    or       'Webrtc::Platform'
31>C:\tdesktop\Telegram\SourceFiles\settings\settings_calls.cpp(417,39): error C2872: 'Platform': ambiguous symbol
31>(compiling source file '../../Telegram/SourceFiles/settings/settings_calls.cpp')
31>    C:\tdesktop\Telegram\SourceFiles\calls\calls_instance.h(16,11):
31>    could be 'Platform'
31>    C:\tdesktop\Telegram\lib_webrtc\webrtc\webrtc_environment.h(15,11):
31>    or       'Webrtc::Platform'
31>C:\tdesktop\Telegram\SourceFiles\settings\settings_calls.cpp(426,62): error C2872: 'Platform': ambiguous symbol
31>(compiling source file '../../Telegram/SourceFiles/settings/settings_calls.cpp')
31>    C:\tdesktop\Telegram\SourceFiles\calls\calls_instance.h(16,11):
31>    could be 'Platform'
31>    C:\tdesktop\Telegram\lib_webrtc\webrtc\webrtc_environment.h(15,11):
31>    or       'Webrtc::Platform'
31>settings_main.cpp

Of course, a better way is to declare it directly at the beginning

using namespace ::Platform;
using namespace Webrtc;