Fix gcc warnings when -Wredundant-decls set

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>

include/mbedtls/constant_time.h

@@ -23,6 +23,10 @@
 
 #include <stddef.h>
 
+#ifdef __GNUC__
+    #pragma GCC diagnostic push
+    #pragma GCC diagnostic ignored "-Wredundant-decls"
+#endif
 
 /** Constant-time buffer comparison without branches.
  *
@@ -43,4 +47,8 @@ int mbedtls_ct_memcmp(const void *a,
                       const void *b,
                       size_t n);
 
+#ifdef __GNUC__
+    #pragma GCC diagnostic pop
+#endif
+
 #endif /* MBEDTLS_CONSTANT_TIME_H */