Fix KTX2 header initialization in included BasisU code (#848)

A change we made to the the subrepo'ed BasisU code as part of the
mass warning fixes in PR #687 broke initialization of the KTX2 header
when BasisU is creating a .ktx2 file. This code is not used by libktx.
The fix is being committed to avoid inadvertently pushing the broken
code upstream.

The user who encountered this was doing something unsupported by libktx.

lib/basisu/encoder/basisu_comp.cpp

@@ -1820,6 +1820,7 @@ namespace basisu
 		}
 
 		basist::ktx2_header header = {};
+		memset(&header, 0, sizeof(header));
 
 		memcpy(header.m_identifier, basist::g_ktx2_file_identifier, sizeof(basist::g_ktx2_file_identifier));
 		header.m_pixel_width = base_width;