meson: fix egl build for surfaceless

Without this, I get:

 > platform_surfaceless.c:38:10: fatal error: 'loader.h' file not found
 > #include "loader.h"
 >      ^~~~~~~~~~
 > 1 error generated.

Fixes: 108d257a ("meson: build libEGL")
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

v2: Split up patches, modify commit message (Dylan)
(cherry picked from commit ec6cb01e)