3rdparty: fix linking of cygQt6Core on CYGWIN

On CYGWIN, the final link of cygQt6Core process fails at this point:

qtbase/src/corelib/CMakeFiles/Core.dir/io/qprocess_unix.cpp.o:qprocess_unix.:(.rdata$.refptr.__interceptor_vfork[.refptr.__interceptor_vfork]+0x0): undefined reference to `__interceptor_vfork'
collect2: error: ld returned 1 exit status

so we just avoid that.

Change-Id: Iafe935981b507aa1449ed41e89fe6e86b9298dde
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qprocess_unix.cpp

@@ -689,6 +689,10 @@ inline bool globalUsingVfork() noexcept
     // why: without the tools to investigate why it happens, we didn't bother.
     return false;
 #endif
+#if defined(Q_OS_CYGWIN)
+    // Fails to link Qt6Core, so we avoid that..
+    return false;
+#endif
 
     // Dynamically detect whether libasan or libtsan are loaded into the
     // process' memory. We need this because the user's code may be compiled