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.

test/integration/smallocx.c

@@ -1,5 +1,12 @@
 #include "test/jemalloc_test.h"
 
+typedef struct {
+	void *ptr;
+	size_t size;
+} smallocx_return_t;
+
+extern smallocx_return_t smallocx(size_t size, int flags);
+
 static unsigned
 get_nsizes_impl(const char *cmd) {
 	unsigned ret;