powerpc/book3s: Add missing <linux/sched.h> include

The functions here use struct task_struct fields, so need to import
the full definition from <linux/sched.h>. The <asm/current.h> header
that defines current only forward declares struct task_struct.

Failing to include this <linux/sched.h> header leads to a compilation
error when a translation unit does not also include <linux/sched.h>
indirectly.
Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230616034846.311705-2-bgray@linux.ibm.com

arch/powerpc/include/asm/book3s/64/kup.h

@@ -194,6 +194,7 @@
 #else /* !__ASSEMBLY__ */
 
 #include <linux/jump_label.h>
+#include <linux/sched.h>
 
 DECLARE_STATIC_KEY_FALSE(uaccess_flush_key);