meson: Fix build for egl platform_x11 with dri3

meson: Fix build for egl platform_x11 with dri3

platform_x11 with dri3 needs inc_loader.

In file included from ../src/egl/drivers/dri2/platform_x11_dri3.c:35:0:
../src/egl/drivers/dri2/egl_dri2.h:41:32: fatal error: loader_dri3_helper.h: No such file or directory
In file included from ../src/egl/drivers/dri2/platform_x11.c:46:0:
../src/egl/drivers/dri2/egl_dri2.h:41:32: fatal error: loader_dri3_helper.h: No such file or directory
In file included from ../src/egl/drivers/dri2/egl_dri2.c:61:0:
../src/egl/drivers/dri2/egl_dri2.h:41:32: fatal error: loader_dri3_helper.h: No such file or directory

Cc: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>