Rename mbedtls_platform_gmtime() to mbedtls_platform_gmtime_r()

For consistency, also rename MBEDTLS_PLATFORM_GMTIME_ALT to
MBEDTLS_PLATFORM_GMTIME_R_ALT.
diff --git a/include/mbedtls/threading.h b/include/mbedtls/threading.h
index 9235a1e..66f78f5 100644
--- a/include/mbedtls/threading.h
+++ b/include/mbedtls/threading.h
@@ -107,7 +107,7 @@
 #if defined(MBEDTLS_FS_IO)
 extern mbedtls_threading_mutex_t mbedtls_threading_readdir_mutex;
 #endif
-#if defined(MBEDTLS_HAVE_TIME_DATE) && !defined(MBEDTLS_PLATFORM_GMTIME_ALT)
+#if defined(MBEDTLS_HAVE_TIME_DATE) && !defined(MBEDTLS_PLATFORM_GMTIME_R_ALT)
 #if !defined(_WIN32) && (defined(unix) || \
     defined(__unix) || defined(__unix__) || (defined(__APPLE__) && \
     defined(__MACH__)))
@@ -122,7 +122,7 @@
 #endif /* !_POSIX_VERSION || 200112L > _POSIX_THREAD_SAFE_FUNCTIONS */
 #endif /* !_WIN32 && (unix || __unix || __unix__ ||
         * (__APPLE__ && __MACH__)) */
-#endif /* MBEDTLS_HAVE_TIME_DATE && !MBEDTLS_PLATFORM_GMTIME_ALT */
+#endif /* MBEDTLS_HAVE_TIME_DATE && !MBEDTLS_PLATFORM_GMTIME_R_ALT */
 #endif /* MBEDTLS_THREADING_C */
 
 #ifdef __cplusplus