Update pixel_buffer_lock.h



fix undeclared identifier

/usr/local/include/ClanLib-3.0/ClanLib/Display/Image/pixel_buffer_lock.h:119:11: error: use of undeclared identifier 'Exception'
throw Exception("Incorrect PixelBufferLock constructor called with a GPU pixelbuffer");
Merge pull request #12 from punkkeks/patch-2

Update pixel_buffer_lock.h