Add local directory to include search path for meson

All includes of usbredir-related headers use the form #include "usbredirparser.h", i.e. not relative to the root directory. As a consequence builds would fail to find files if they weren't installed in a global include directory:

../usbredirhost/usbredirhost.h:25:10: fatal error: usbredirparser.h: No such file or directory

Adding "." to the include paths avoids this issue.