Add strict requirement on Qt version

Http2DirectAttribute appeared since Qt v.5.11:
https://doc.qt.io/qt-5/whatsnew511.html

The qtprotobuf is required to be built
with Qt 5.12+ as first LTS release after 5.11

src/lib/CMakeLists.txt

@@ -1,4 +1,7 @@
 find_package(Qt5 COMPONENTS Core Network REQUIRED)
+if (Qt5Core_VERSION VERSION_LESS "5.12.0")
+    message(FATAL_ERROR "Required Qt version is 5.12+")
+endif()
 
 set(SUPPORT_LIBRARY_TARGET qtprotobufsupport)