bpf: add missing header file include

Commit 74e19ef ("uaccess: Add speculation barrier to
copy_from_user()") built fine on x86-64 and arm64, and that's the extent
of my local build testing.

It turns out those got the <linux/nospec.h> include incidentally through
other header files (<linux/kvm_host.h> in particular), but that was not
true of other architectures, resulting in build errors

  kernel/bpf/core.c: In function ‘___bpf_prog_run’:
  kernel/bpf/core.c:1913:3: error: implicit declaration of function ‘barrier_nospec’

so just make sure to explicitly include the proper <linux/nospec.h>
header file to make everybody see it.

Fixes: 74e19ef ("uaccess: Add speculation barrier to copy_from_user()")
Reported-by: kernel test robot <[email protected]>
Reported-by: Viresh Kumar <[email protected]>
Reported-by: Huacai Chen <[email protected]>
Tested-by: Geert Uytterhoeven <[email protected]>
Tested-by: Dave Hansen <[email protected]>
Acked-by: Alexei Starovoitov <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: torvalds

kernel/bpf/core.c

@@ -34,6 +34,7 @@
 #include <linux/log2.h>
 #include <linux/bpf_verifier.h>
 #include <linux/nodemask.h>
+#include <linux/nospec.h>
 #include <linux/bpf_mem_alloc.h>
 #include <linux/memcontrol.h>