x86/ftrace: Explicitly include vmalloc.h for set_vm_flush_reset_perms()

The inclusion of linux/vmalloc.h, which is required for its definition
of set_vm_flush_reset_perms(), is somehow dependent on asm/realmode.h
being included by asm/acpi.h.  Explicitly include linux/vmalloc.h so
that a future patch can drop the realmode.h include from asm/acpi.h
without breaking the build.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: https://lkml.kernel.org/r/20191126165417.22423-4-sean.j.christopherson@intel.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/ftrace.c

@@ -23,6 +23,7 @@
 #include <linux/list.h>
 #include <linux/module.h>
 #include <linux/memory.h>
+#include <linux/vmalloc.h>
 
 #include <trace/syscall.h>