x86/entry/vdso: Fix path of included gettimeofday.c

Commit in Fixes forgot to convert one include path to be relative to the
kernel source directory after adding latter to flags-y.

Fix it.

  [ bp: Rewrite commit message. ]

Fixes: 693c819 ("x86/entry/vdso: Refactor the vdso build")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20260307174406.1808981-1-vladimir.oltean@nxp.com

Author: vladimiroltean

arch/x86/entry/vdso/common/vclock_gettime.c

@@ -13,7 +13,7 @@
 #include <linux/types.h>
 #include <vdso/gettime.h>
 
-#include "../../../../lib/vdso/gettimeofday.c"
+#include "lib/vdso/gettimeofday.c"
 
 int __vdso_gettimeofday(struct __kernel_old_timeval *tv, struct timezone *tz)
 {