hw/xfree86: Fix -Wincompatible-pointer-types sbus compile failure
``` ../hw/xfree86/common/xf86sbusBus.c: In function ‘xf86SbusConfigureNewDev’: ../hw/xfree86/common/xf86sbusBus.c:751:21: error: passing argument 1 of ‘XNFasprintf’ from incompatible pointer type [-Wincompatible-pointer-types] 751 | XNFasprintf(&GDev->busID, "SBUS:%s", promPath); | ^~~~~~~~~~~~ | | | const char ** ``` Apply the same fix as in commit e1e01d2e ("xfree86/common: Warning fixes. Mostly const string handling.") (cherry picked from commit bdacb100) Part-of: <!1752>