package/x11r7/xserver_xorg-server: Add libdrm dependency when dri or dri2 is enabled

Fixes compile error using this defconfig
BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
BR2_TOOLCHAIN_BUILDROOT_CXX=y
BR2_PACKAGE_XORG7=y
BR2_PACKAGE_XSERVER_XORG_SERVER=y
BR2_PACKAGE_XPROTO_DRI2PROTO=y

drmVersionPtr is referenced not only in hw/xfree86/dri2/dri2.c
but also in hw/xfree86/dri/dri.c.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>