Fix gcc -Wliteral-suffix warning

Add extra whitespace in png.h to fix gcc warning:

VTK/ThirdParty/png/vtkpng/png.h:2217:29: warning: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wliteral-suffix]
      fprintf(PNG_DEBUG_FILE,"%s"m,(num_tabs==1 ? "\t" : \
                             ^

I have not checked or synced with upstream
partly because there isn't even a readme
that tells me what version of libpng this is
or what custom changes have been made, if any.

Change-Id: I7ea3b0787e9c62964ca297ad73dcc15afaa00852
Merge topic 'png-warnings' into master

0482ec09 Fix gcc -Wliteral-suffix warning