meson: add missing libweston EGL dependency if renderer-gl is enabled

If the 'renderer-gl' option is enabled, ENABLE_EGL is defined, and
libweston/pixel-formats.c includes EGL/egl.h. This requires an egl
dependency, as X11-less platforms need the MESA_EGL_NO_X11_HEADERS
define from egl.pc cflags:

  In file included from /usr/include/EGL/egl.h:39:0,
                   from ../libweston/pixel-formats.c:42:
  /usr/include/EGL/eglplatform.h:124:10: fatal error: X11/Xlib.h: No such file or directory
  compilation terminated.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>