xquartz: fix incomplete prototype of executable_path()

> ../hw/xquartz/mach-startup/bundle_trampoline.c:53:29: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
> static char *executable_path() {
>                             ^
>                              void
> 1 warning generated.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

hw/xquartz/mach-startup/bundle_trampoline.c

@@ -50,7 +50,7 @@
  * needs and simply execs the startup script which then execs the main binary.
  */
 
-static char *executable_path() {
+static char *executable_path(void) {
     uint32_t bufsize = PATH_MAX;
     char *buf = calloc(1, bufsize);