Use non-empty definition for fallback

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/common.h b/library/common.h
index 3e81e81..7652e56 100644
--- a/library/common.h
+++ b/library/common.h
@@ -330,7 +330,7 @@
 #endif
 #endif
 #if !defined(MBEDTLS_ASSUME)
-#define MBEDTLS_ASSUME(x)
+#define MBEDTLS_ASSUME(x)       do { } while (0)
 #endif
 
 #if defined(__GNUC__) && !defined(__ARMCC_VERSION) && !defined(__clang__) \