Fix #45: Missing braces in for loop

[Zenexer]

Fixes #45

[davemevans]

h264_sei.c is supposed to be auto-generated from h264_sei.in.c, so this fix should also be done at https://github.com/aizvorski/h264bitstream/blob/master/h264_sei.in.c#L270, which is the real cause of the issue.

I offered up the same fix in https://github.com/aizvorski/h264bitstream/pull/40/files#diff-eae43aaacca503a3845324c0c595892462d12b3989b8cdf23d3204e42d51dc04R433, but this has sat for 15 months, so I don't expect it to be merged any time soon.