log: Move SEVERITY_MASK_IDX() to log.h

Move SEVERITY_MASK_IDX() to log.h private/unit tests section, so that
we can use it in log.c, the unit tests, and the fuzzers.

(The test and fuzzer code changes are in a subsequent commit.)

Preparation for bug 31334.

src/lib/log/log.c

@@ -55,10 +55,6 @@
 #include <android/log.h>
 #endif // HAVE_ANDROID_LOG_H.
 
-/** Given a severity, yields an index into log_severity_list_t.masks to use
- * for that severity. */
-#define SEVERITY_MASK_IDX(sev) ((sev) - LOG_ERR)
-
 /** @{ */
 /** The string we stick at the end of a log message when it is too long,
  * and its length. */

src/lib/log/log.h

@@ -297,4 +297,10 @@ MOCK_DECL(STATIC void, logv, (int severity, log_domain_mask_t domain,
     va_list ap) CHECK_PRINTF(5,0));
 #endif
 
+#if defined(LOG_PRIVATE) || defined(TOR_UNIT_TESTS)
+/** Given a severity, yields an index into log_severity_list_t.masks to use
+ * for that severity. */
+#define SEVERITY_MASK_IDX(sev) ((sev) - LOG_ERR)
+#endif
+
 #endif /* !defined(TOR_TORLOG_H) */