mark _pthread_mutex_init_calloc_cb as public explicitly

[Jan Beich]

Mozilla build hides everything by default using visibility pragma and
unhides only explicitly listed headers. But this doesn't work on FreeBSD
because _pthread_mutex_init_calloc_cb is neither documented nor exposed
via any header.

diff --git a/src/mutex.c b/src/mutex.c
index 37a843e..55e18c2 100644
--- a/src/mutex.c
+++ b/src/mutex.c
@@ -64,7 +64,7 @@ pthread_create(pthread_t *__restrict thread,
 /******************************************************************************/
 
 #ifdef JEMALLOC_MUTEX_INIT_CB
-int	_pthread_mutex_init_calloc_cb(pthread_mutex_t *mutex,
+JEMALLOC_EXPORT int	_pthread_mutex_init_calloc_cb(pthread_mutex_t *mutex,
     void *(calloc_cb)(size_t, size_t));
 #endif