mesa3d-headers: fix logic to generate the dri.pc file

mesa3d-headers: fix logic to generate the dri.pc file

As noted by Arnout in [1], the logic in mesa3d-headers.mk generates a
bogus dri.pc file, which looks like this:

prefix=/usr
exec_prefix=/usr
libdir=/lib
includedir=/include
dridriverdir=/dri

Indeed, the ${...} are expanded as shell variables when the sed
command is executed, while the intention is that those ${...} should
go in the .pc file. By escaping those using $${...}, we get the
expected .pc file:

prefix=/usr
exec_prefix=/usr
libdir=${exec_prefix}/lib
includedir=${prefix}/include
dridriverdir=${libdir}/dri

This was detected by the not yet committed check-package improvement
from Ricardo that detects bogus ${...} usage to reference make
variables.

[1] http://lists.busybox.net/pipermail/buildroot/2018-July/225402.html

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>