meson: fix build for egl platform_x11 without dri3 and gbm

Compiling EGL's platform_x11 without dri3 and gbm yields this compile
failure:

platform_x11 needs inc_loader:

../mesa-18.2.0-rc2/src/egl/drivers/dri2/platform_x11.c:48:10: fatal
error: loader.h: No such file or directory
 #include "loader.h"
          ^~~~~~~~~~

Fixes: 108d257a ("meson: build libEGL")
Bugzilla: https://bugs.gentoo.org/663534


Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit 9a96bf0e)