Add missing libdrm dev packages to firefox config

We fixed Wayland support with the switch to Jessie (see: #31729 (closed)). However, that might not have been enough. I still get a related warning in the configure script it seems:

 0:14.59 checking for gtk+-wayland-3.0 >= 3.10 xkbcommon >= 0.4.1 libdrm
>= 2.4... no
 0:14.59 WARNING: Package libdrm was not found in the pkg-config search
path.
 0:14.59 WARNING: Perhaps you should add the directory containing
`libdrm.pc'
 0:14.59 WARNING: to the PKG_CONFIG_PATH environment variable
 0:14.59 WARNING: No package 'libdrm' found

So we should add the respective libdrm packages, too. See: https://bugzilla.mozilla.org/show_bug.cgi?id=1648555.