Update the list of required Tiles packages (Cgettys)

More recent versions of Debian have started to require the installation
of libglu1-mesa and libglu1-mesa-dev to get GL/glu.h.

I'll not here that some cursory searches about this header point to it
being part of the OpenGL utilities library, which is something that is
no longer maintained and should no longer be used. Updating the OpenGL
portion of our Tile code is not part of my skill set, so for now I'll
just include the packages we need to get the headers we need for
compilation. Long term, we should probably look to modernising our
OpenGL code, since functions we're depending on may not be around much
longer.

Closes #4200, closes #3998.

crawl-ref/INSTALL.md

@@ -83,7 +83,8 @@ libsqlite3-dev libz-dev pkg-config python3-yaml binutils-gold python-is-python3
 
 # Dependencies for tiles builds
 sudo apt install libsdl2-image-dev libsdl2-mixer-dev libsdl2-dev \
-libfreetype6-dev libpng-dev fonts-dejavu-core advancecomp pngcrush
+libfreetype6-dev libglu1-mesa libglu1-mesa-dev libpng-dev fonts-dejavu-core \
+advancecomp pngcrush
 ```
 
 Then follow [the above compilation steps](#compiling).