wayland-egl: Ignore underscored symbols in ABI check

Rather than a hard-coded list of platform symbols, just ignore anything
prefaced with an underscore. This fixes breakage on ARM, which declares
several slightly different platform symbols to x86.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Fixes: 21b1f22e ("wayland-egl: enhance the symbol test")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105620


Cc: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>