CMakeLists: avoid adding/compiling sources twice in static lib

[obligaron]

When compiling as static library the "main" sources (libztSrcGlob) are added to zt_pic and the static library itself.
The static library on the other hand also includes zt_pic. That means the code is included twice and MSVC warns about it (LNK4006).
This PR removes the second include/compilation.

When compiling with MSVC this PR removes 256 warnings.
Found in devilutionX (see also related discussion).

Example warnings:

VirtualTap.cpp.obj : warning LNK4006: "bool ZeroTier::_has_exited" (?_has_exited@ZeroTier@@3_NA) already defined in VirtualTap.cpp.obj; second definition ignored
VirtualTap.cpp.obj : warning LNK4006: "bool ZeroTier::_has_started" (?_has_started@ZeroTier@@3_NA) already defined in VirtualTap.cpp.obj; second definition ignored
VirtualTap.cpp.obj : warning LNK4006: "int ZeroTier::netifCount" (?netifCount@ZeroTier@@3Ha) already defined in VirtualTap.cpp.obj; second definition ignored
VirtualTap.cpp.obj : warning LNK4006: "public: __cdecl ZeroTier::VirtualTap::VirtualTap(char const ,class ZeroTier::MAC const &,unsigned int,unsigned int,unsigned __int64,void (__cdecl)(void *,void *,unsigned __int64,class ZeroTier::MAC const &,class ZeroTier::MAC const &,unsigned int,unsigned int,void const *,unsigned int),void *)" (??0VirtualTap@ZeroTier@@qeaa@PEBDAEBVMAC@1@II_KP6AXPEAX3211IIPEBXI@Z3@Z) already defined in VirtualTap.cpp.obj; second definition ignored
VirtualTap.cpp.obj : warning LNK4006: "public: __cdecl ZeroTier::VirtualTap::~VirtualTap(void)" (??1VirtualTap@ZeroTier@@qeaa@XZ) already defined in VirtualTap.cpp.obj; second definition ignored
VirtualTap.cpp.obj : warning LNK4006: "public: void __cdecl ZeroTier::VirtualTap::setEnabled(bool)" (?setEnabled@VirtualTap@ZeroTier@@QEAAX_N@Z) already defined in VirtualTap.cpp.obj; second definition ignored
VirtualTap.cpp.obj : warning LNK4006: "public: bool __cdecl ZeroTier::VirtualTap::enabled(void)const "

[joseph-henry]

Thanks for catching this. This does appear to be some sort of copy and paste error since it makes no sense for it to be there. I don't have MSVC handy but I did test this on Linux and it doesn't cause any new issues.