Use correct test case conditionals for helper functions in tests/suites

Fix unused-function errors.

Signed-off-by: Michael Schuster <michael@schuster.ms>
diff --git a/tests/suites/test_suite_bignum.function b/tests/suites/test_suite_bignum.function
index 7cbbbe8..1830e5a 100644
--- a/tests/suites/test_suite_bignum.function
+++ b/tests/suites/test_suite_bignum.function
@@ -32,6 +32,7 @@
     return 1;
 }
 
+#if defined(MBEDTLS_GENPRIME)
 typedef struct mbedtls_test_mpi_random {
     data_t *data;
     size_t  pos;
@@ -73,6 +74,7 @@
 
     return 0;
 }
+#endif /* MBEDTLS_GENPRIME */
 
 /* Random generator that is told how many bytes to return. */
 static int f_rng_bytes_left(void *state, unsigned char *buf, size_t len)