Since building libweston includes EGL/egl.h from pixel-formats.c,
EGL_CFLAGS must be added to libweston_CFLAGS, as on some platforms that
contains -DMESA_EGL_NO_X11_HEADERS, and fails to compile without it:

  CC       libweston/libweston_3_la-pixel-formats.lo
In file included from [...]/usr/include/EGL/egl.h:39:0,
                 from libweston/pixel-formats.c:39:
[...]/usr/include/EGL/eglplatform.h:119:22:
fatal error: X11/Xlib.h: No such file or directory

Fixes: 903721a6 ("libweston: Add pixel-format helpers")
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Daniel Stone <daniels@collabora.com>