h264: Make it possible to compile without error_resilience

Error resilience is enabled by the h264 decoder, unless explicitly
disabled. --disable-everything --enable-decoder=h264 will produce
a h264 decoder with error resilience enabled, while
--disable-everything --enable-decoder=h264 --disable-error-resilience
will produce a h264 decoder with error resilience disabled.

Signed-off-by: Martin Storsjö <martin@martin.st>