generate_visualc_files.pl has a list of directories that it pulls
headers from, so it knows what directories to put on the include path.
Make it inject the include path into the output files, rather than
hard-coding the include paths in template files.
A similar change (but with different code) was made in Mbed TLS in
commit b78cf2b261
"Adjust visual studio file generation to always use the crypto submodule".
No semantic change: this commit does not change the generated files.
7156d8cda9