ArmPkg: Fix Ecc error 6001 in ArmSoftFloatLib

This patch fixes the following Ecc reported error:
All include file contents should be guarded by
a #ifndef statement.

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>

ArmPkg/Library/ArmSoftFloatLib/platform.h

@@ -1,11 +1,17 @@
 /*
  * Copyright (c) 2019, Linaro Limited
+ * Copyright (c) 2021, Arm Limited. All rights reserved.<BR>
  *
  * SPDX-License-Identifier: BSD-2-Clause-Patent
  */
 
+#ifndef ARM_SOFT_FLOAT_LIB_H_
+#define ARM_SOFT_FLOAT_LIB_H_
+
 #define LITTLEENDIAN 1
 #define INLINE static inline
 #define SOFTFLOAT_BUILTIN_CLZ 1
 #define SOFTFLOAT_FAST_INT64
 #include "opts-GCC.h"
+
+#endif // ARM_SOFT_FLOAT_LIB_H_