EGL: Add -DEGL_NO_X11 to egl.pc Cflags when X11 is disabled

This replicates mesa's egl.pc behavior and fixes build issues on systems
without X11.

Bug: https://bugs.webkit.org/show_bug.cgi?id=163483
2 jobs for bugfix/egl_no_x11 in 1 minute and 52 seconds (queued for 17 seconds)
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