Fix builds when config.h only defines MBEDTLS_BIGNUM_C

Fixes #4929

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

include/mbedtls/entropy.h

@@ -124,7 +124,7 @@ typedef struct mbedtls_entropy_context
                               * -1 after free. */
 #if defined(MBEDTLS_ENTROPY_SHA512_ACCUMULATOR)
     mbedtls_sha512_context  MBEDTLS_PRIVATE(accumulator);
-#else
+#elif defined(MBEDTLS_ENTROPY_SHA256_ACCUMULATOR)
     mbedtls_sha256_context  MBEDTLS_PRIVATE(accumulator);
 #endif
     int             MBEDTLS_PRIVATE(source_count); /* Number of entries used in source. */

library/bignum.c

@@ -43,6 +43,7 @@
 #include "mbedtls/error.h"
 #include "constant_time_internal.h"
 
+#include <limits.h>
 #include <string.h>
 
 #if defined(MBEDTLS_PLATFORM_C)

programs/fuzz/common.c

@@ -1,4 +1,5 @@
 #include "common.h"
+#include <limits.h>
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>