_dbus_get_is_errno_eagain_or_ewouldblock: Avoid warning

EAGAIN and EWOULDBLOCK are documented to possibly be numerically equal,
for instance in errno(3), and a simple logical OR check will trigger the
-Wlogical-op warning of GCC. The GCC developers consider the warning to
work as-designed in this case:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69602

Avoid such a warning by explicitly checking if the values are identical.

Fixes: dbus/dbus#225
Signed-off-by: David King <dking@redhat.com>
Reviewed-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit a6531913)
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