meson: fix egl build for meson version < 0.43

meson: fix egl build for meson version < 0.43

Meson 0.43 added the ability to pass nested lists to
include_directories, so the code that we have works for 0.43, but not
for 0.42. This patch changes the include_directories list to be flat so
it works with 0.42

fixes: 108d257a16859898f5c ("meson: build libEGL")
Tested-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Rhys Kidd <rhyskidd@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>