Bug#971529: libsdl2: Please add libdrm-dev to build deps

[Matthew Harm Bekkema]

Source: libsdl2
Version: 2.0.12+dfsg1-2

Dear Maintainer,

I've been trying to use the kmsdrm video driver, but I kept on getting
the error:

> ERROR: SDL_Init failed: kmsdrm not available

I believe adding libdrm-dev to the build deps should fix this. Compare
these two snippets of build output:

> # Before installing libdrm-dev
> Enabled modules : atomic audio video render events joystick haptic sensor power filesystem threads timers file loadso cpuinfo assembly
> Assembly Math   : mmx 3dnow sse sse2 sse3
> Audio drivers   : disk dummy oss alsa pulse sndio(dynamic)
> Video drivers   : dummy x11 opengl opengl_es2 vulkan wayland
> X11 libraries   : xcursor xdbe xinerama xinput2 xinput2_multitouch xrandr xscrnsaver xshape xvidmode
> Input drivers   : linuxev linuxkd
> 
> # After installing libdrm-dev
> Enabled modules : atomic audio video render events joystick haptic sensor power filesystem threads timers file loadso cpuinfo assembly
> Assembly Math   : mmx 3dnow sse sse2 sse3
> Audio drivers   : disk dummy oss alsa pulse sndio(dynamic)
> Video drivers   : dummy x11 kmsdrm opengl opengl_es2 vulkan wayland
> X11 libraries   : xcursor xdbe xinerama xinput2 xinput2_multitouch xrandr xscrnsaver xshape xvidmode
> Input drivers   : linuxev linuxkd

Best regards,
Matthew