egl: add missing #include <stddef.h> in egldevice.h

egl: add missing #include <stddef.h> in egldevice.h

Otherwise, I get this error:

main/egldevice.h:54:13: error: ‘NULL’ undeclared (first use in this function)
       dev = NULL;
             ^~~~
with this config:

./autogen.sh --enable-gles1 --enable-gles2 --with-platforms='surfaceless' --disable-glx
             --with-dri-drivers="i965" --with-gallium-drivers="" --enable-gbm

v3: Use stddef.h (Matt)
v4: Modify commit message (Eric)

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>