Hide smallocx even when enabled from the library API

The experimental `smallocx` API is not exposed via header files,
requiring the users to peek at `jemalloc`'s source code to manually
add the external declarations to their own programs.

This should reinforce that `smallocx` is experimental, and that `jemalloc`
does not offer any kind of backwards compatiblity or ABI gurantees for it.

src/jemalloc.c

@@ -2770,6 +2770,11 @@ int __posix_memalign(void** r, size_t a, size_t s) PREALIAS(je_posix_memalign);
  */
 
 #ifdef JEMALLOC_EXPERIMENTAL_SMALLOCX_API
+typedef struct {
+	void *ptr;
+	size_t size;
+} smallocx_return_t;
+
 JEMALLOC_EXPORT JEMALLOC_ALLOCATOR JEMALLOC_RESTRICT_RETURN
 smallocx_return_t JEMALLOC_NOTHROW
 /*