wpa_supplicant: Fix couple of C++ compiler errors with header files

wpa_supplicant: Fix couple of C++ compiler errors with header files

Need to include these headers in C++ files for adding a binder interface
to wpa_supplicant. So, fix the following C++ compiler errors in them:
1. Add explicit C-style casts in wpa_buf.h header.
2. Move the nested definition of wpa_driver_scan_ssid in driver.h
outside of wpa_driver_scan_params because it is used in another
structure below.

Signed-off-by: Roshan Pius <rpius@google.com>