Fix compilation error when ENABLE_PIE=1

This patch fixes compilation errors when ENABLE_PIE=1.

<snip>
bl31/aarch64/bl31_entrypoint.S: Assembler messages:
bl31/aarch64/bl31_entrypoint.S:61: Error: invalid operand (*UND* section) for `~'
bl31/aarch64/bl31_entrypoint.S:61: Error: invalid immediate
Makefile:1079: recipe for target 'build/tegra/t194/debug/bl31/bl31_entrypoint.o' failed
<snip>

Verified by setting 'ENABLE_PIE=1' for Tegra platform builds.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>
Change-Id: Ifd184f89b86b4360fda86a6ce83fd8495f930bbc

include/arch/aarch64/el3_common_macros.S

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015-2019, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2015-2020, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -9,6 +9,7 @@
 
 #include <arch.h>
 #include <asm_macros.S>
+#include <lib/xlat_tables/xlat_tables_defs.h>
 
 	/*
 	 * Helper macro to initialise EL3 registers we care about.