Added compile_commands.json to Qt.gitignore

QtCreator above 4.8 version is utilizing a so called "compilation database" which placed in the compile_commands.json and contains generated information.
2019-01-17 17:39:58 +01:00 · 2019-01-17 17:39:58 +01:00 · 1c1744ab6d
--- a/Qt.gitignore
+++ b/Qt.gitignore
@ -41,3 +41,6 @@ target_wrapper.*

 # QtCreator CMake
 CMakeLists.txt.user*
+
+# QtCreator 4.8< compilation database 
+compile_commands.json