virmocklibxl.c: Fix include of virfile.h

Firstly, this isn't supposed to be in angle brackets because it's
not a system header file (not that gcc distinguishes these two,
it's just guide for us developers).

Secondly, no need to use util/ path prefix because CFLAGS already
contain -I$(srcdir)/util.

The include is needed because virmocklibxl is mocking
virFileMakePath() and even when VIR_MOCK_IMPL_RET_ARGS() does
forward declaration to shut up compiler we need real
virFileMakePath() declaration so that when it changes compiler
catches mismatching declarations.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

tests/virmocklibxl.c

@@ -27,11 +27,12 @@
 # include <sys/stat.h>
 # include <unistd.h>
 # include <libxl.h>
-# include <util/virfile.h>
 # include <xenstore.h>
 # include <xenctrl.h>
 # include <sys/socket.h>
 
+# include "virfile.h"
+
 VIR_MOCK_IMPL_RET_VOID(xs_daemon_open,
                        struct xs_handle *)
 {