Fix missing-prototype errors in tests/suites

Signed-off-by: Michael Schuster <michael@schuster.ms>
diff --git a/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index 1db7e1c..820b48d 100644
--- a/tests/suites/test_suite_x509write.function
+++ b/tests/suites/test_suite_x509write.function
@@ -11,14 +11,14 @@
 #include "mbedtls/psa_util.h"
 
 #if defined(MBEDTLS_RSA_C)
-int mbedtls_rsa_decrypt_func(void *ctx, size_t *olen,
+static int mbedtls_rsa_decrypt_func(void *ctx, size_t *olen,
                              const unsigned char *input, unsigned char *output,
                              size_t output_max_len)
 {
     return mbedtls_rsa_pkcs1_decrypt((mbedtls_rsa_context *) ctx, NULL, NULL,
                                      olen, input, output, output_max_len);
 }
-int mbedtls_rsa_sign_func(void *ctx,
+static int mbedtls_rsa_sign_func(void *ctx,
                           int (*f_rng)(void *, unsigned char *, size_t), void *p_rng,
                           mbedtls_md_type_t md_alg, unsigned int hashlen,
                           const unsigned char *hash, unsigned char *sig)
@@ -26,7 +26,7 @@
     return mbedtls_rsa_pkcs1_sign((mbedtls_rsa_context *) ctx, f_rng, p_rng,
                                   md_alg, hashlen, hash, sig);
 }
-size_t mbedtls_rsa_key_len_func(void *ctx)
+static size_t mbedtls_rsa_key_len_func(void *ctx)
 {
     return ((const mbedtls_rsa_context *) ctx)->len;
 }