x86/vdso: Conditionally export __vdso_sgx_enter_enclave()

Recently, ld.lld moved from '--undefined-version' to
'--no-undefined-version' as the default, which breaks building the vDSO
when CONFIG_X86_SGX is not set:

  ld.lld: error: version script assignment of 'LINUX_2.6' to symbol '__vdso_sgx_enter_enclave' failed: symbol not defined

__vdso_sgx_enter_enclave is only included in the vDSO when
CONFIG_X86_SGX is set. Only export it if it will be present in the final
object, which clears up the error.

Fixes: 84664369 ("x86/vdso: Implement a vDSO for Intel SGX enclave call")                                                                                                                                                               
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/1756
Link: https://lore.kernel.org/r/20221109000306.1407357-1-nathan@kernel.org

arch/x86/entry/vdso/vdso.lds.S

@@ -27,7 +27,9 @@ VERSION {
 		__vdso_time;
 		clock_getres;
 		__vdso_clock_getres;
+#ifdef CONFIG_X86_SGX
 		__vdso_sgx_enter_enclave;
+#endif
 	local: *;
 	};
 }