Lock duchain in AbstractIncludeNavigationContext::html

Fixes assertion when trying to navigate in project file
quick open navigation widget:

#9  0x00007fc05e472c28 in qt_assert(char const*, char const*, int) () from /usr/lib/libQt5Core.so.5
#10 0x00007fc061696c26 in KDevelop::TopDUContext::importedParentContexts (this=0x561284615030) at /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/language/duchain/topducontext.cpp:1015
#11 0x00007fc0617883cc in KDevelop::pickContextWithData (duchains=..., maxDepth=2, type=@0x561287177ac0: KDevelop::StandardParsingEnvironment, forcePick=true) at /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/language/duchain/navigation/abstractincludenavigationcontext.cpp:60
#12 0x00007fc061788815 in KDevelop::AbstractIncludeNavigationContext::html (this=0x561287177aa0, shorten=false) at /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/language/duchain/navigation/abstractincludenavigationcontext.cpp:90
#13 0x00007fc0617752fb in KDevelop::AbstractNavigationContext::down (this=0x561287177aa0) at /home/milian/projects/kf5/src/extragear/kdevelop/kdevelop/kdevplatform/language/duchain/navigation/abstractnavigationcontext.cpp:282