README.md: Add qtbase5-private-dev requirement for Linux

README.md

@@ -87,7 +87,7 @@ Requirements (Debian/Ubuntu package names):
  - CMake (`cmake`)
  - SDL2 (`libsdl2-dev`)
  - GTK2.0 for file selector (`libgtk2.0-dev`)
- - Qt 5 (`qtbase5-dev`, `qtbase5-dev-tools`)
+ - Qt 5 (`qtbase5-dev`, `qtbase5-private-dev`, `qtbase5-dev-tools`)
  - Optional for faster building: Ninja (`ninja-build`)
 
 1. Clone the repository. Submodules aren't necessary, there is only one and it is only used for Windows.