bpf: fix perf_snapshot_branch_stack link failure

The newly added code to handle bpf_get_branch_snapshot fails to link when
CONFIG_PERF_EVENTS is disabled:

aarch64-linux-ld: kernel/bpf/verifier.o: in function `do_misc_fixups':
verifier.c:(.text+0x1090c): undefined reference to `__SCK__perf_snapshot_branch_stack'

Add a build-time check for that Kconfig symbol around the code to
remove the link time dependency.

Fixes: 314a53623cd4 ("bpf: inline bpf_get_branch_snapshot() helper")
Signed-off-by: Arnd Bergmann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/verifier.c

···
 		}
 
 		/* Implement bpf_get_branch_snapshot inline. */
-		if (prog->jit_requested && BITS_PER_LONG == 64 &&
+		if (IS_ENABLED(CONFIG_PERF_EVENTS) &&
+		    prog->jit_requested && BITS_PER_LONG == 64 &&
 		    insn->imm == BPF_FUNC_get_branch_snapshot) {
 			/* We are dealing with the following func protos:
 			 * u64 bpf_get_branch_snapshot(void *buf, u32 size, u64 flags);