[mythtv] One line fix for gcc 3.4.1 (mythvideo)

[Brandon Beattie]

While trying Myth with gcc 3.4.1, there was a problem compiling
mythvideo plugin.  

videogallery.cpp:248: error: `ceilf' undeclared (first use this function)

This can be fixed by adding 

#include <math.h> 

to mythvideo/videogallery.cpp . 

I did notice about a 7% increase performance in decoding video with gcc
3.4.1, which is good news.  I can finally decode 1080i in software again
with almost no stuttering.

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