codeview: Look up %include path when determining files to hash.

codeview: Look up %include path when determining files to hash.

The hash calculation in calc_md5 tries to open the source file via
"filename" again. For %includes, this is the file name that was
specified in the %include directive, not the actual name of the file
that was opened by the preprocessor. In other words, this fails if the
include file is not in the current working directory.

Add pp_input_fopen that uses the preprocessor include path lookup
code to resolve a file name and open it, and use that in codeview.c.

Signed-off-by: Fabian Giesen <fabiang@radgametools.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>