Put jl_gc_new_weakref in a header file again (#56319)

Author: lgoettgens

src/gc-common.h

@@ -185,4 +185,13 @@ extern jl_ptls_t* gc_all_tls_states;
 
 extern int gc_logging_enabled;
 
+// =========================================================================== //
+// Misc
+// =========================================================================== //
+
+// Allocates a new weak-reference, assigns its value and increments Julia allocation
+// counters. If thread-local allocators are used, then this function should allocate in the
+// thread-local allocator of the current thread.
+JL_DLLEXPORT jl_weakref_t *jl_gc_new_weakref(jl_value_t *value);
+
 #endif // JL_GC_COMMON_H