Fix nasm includes

[bsdinis]

When building for Linux, nasm include directories come without a trainling slash;
This goes against the nasm man page:

 -I|-i directory
           Adds a directory to the search path for include files. The
           directory specification must include the trailing slash, as
           it will be directly prepended to the name of the include
           file.

[amatyuko]

This is probably a bug in the NASM documentation, as trailing slashes are not required as of NASM 2.14.
See NASM bug report: https://bugzilla.nasm.us/show_bug.cgi?id=3392205

Regards,
Andrey