bugfix: do not interact with dbus directory if dbus proxy is disabled

Fix a failing assert when running `firejail` inside:
`firejail --chroot=somedir --noprofile`

firejail: ../../src/firejail/util.c:1039: create_empty_dir_as_root:
Assertion `(s.st_mode & 07777) == (mode)' failed.

The assert is checking whether the newly created `/run/firejail/dbus`
empty directory, has the correct mode set, which in this case is `0755`.
The issue is that, since the directory already exists and has the mode
`0400`, because of the first ran chroot command, the assert fails.

This commit circumvents this issue, by making `firejail` ignore the dbug
directory entirely, in the `src/firejail/preproc.c` file when compiling
the project using `configure --disable-dbusproxy`.
There's also really no need for `firejail` to care about that directory
if `--disable-dbusproxy` is used.

src/firejail/preproc.c

@@ -142,8 +142,10 @@ void preproc_build_firejail_dir_locked(void) {
 	// only root should be able to lock files in this directory
 	create_empty_dir_as_root(RUN_FIREJAIL_SANDBOX_DIR, 0700);
 
+#ifdef HAVE_DBUSPROXY
 	create_empty_dir_as_root(RUN_FIREJAIL_DBUS_DIR, 0755);
 	fs_remount(RUN_FIREJAIL_DBUS_DIR, MOUNT_NOEXEC, 0);
+#endif
 
 	create_empty_dir_as_root(RUN_RO_DIR, S_IRUSR);
 	fs_remount(RUN_RO_DIR, MOUNT_READONLY, 0);