Fix VGG::getDefaultName unresolved external symbol link error

[TheNetos]

Greetings!

When compiling DSO xfeatures2d with option OPENCV_SKIP_FEATURES2D_DOWNLOADING=ON on Windows, a problem was discovered:

Creating library OpenCV-Release-build/lib/Release/opencv_xfeatures2d4120.lib and object OpenCV-Release-build/lib/Release/opencv_xfeatures2d4120.exp
error LNK2019: unresolved external symbol "public: virtual class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl cv::xfeatures2d::VGG::getDefaultName(void)const " (?getDefaultName@VGG@xfeatures2d@cv@@UEBA?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@XZ) referenced in function "[thunk]:public: virtual class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl cv::xfeatures2d::VGG::getDefaultName`vtordisp{4294967292,0}' (void)const " (?getDefaultName@VGG@xfeatures2d@cv@@$4PPPPPPPM@A@EBA?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@XZ) [OpenCV-Release-build\modules\xfeatures2d\opencv_xfeatures2d.vcxproj] [OpenCV-Release.vcxproj]
OpenCV-Release-build\bin\Release\opencv_xfeatures2d4120.dll : fatal error LNK1120: 1 unresolved externals

However, this problem does not exist in the boostdesc implementation, which is built on a similar principle.

This patch fixes this behavior by moving the implementation of function VGG::getDefaultName() out of the macro OPENCV_XFEATURES2D_HAS_VGG_DATA scope.

This bug affects all versions starting with 4.7.0 and up to the current 4.x and 5.x.

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